FEB  In  m^ 


LIBRARY 

OF  THE 

University  OF  California. 

GIFT    OF 

U^.% u^^.o^^ (LuJr- 

Class                   ^ 

WAR  DEPARTMENT 
OFFICE  OF  THE  CHIEF  SIGNAL  OFFICER 


MANUAL  No.  6 


VISUAL  SIGNALING 

SIGNAL  CORPS 
UNITED  STATES  ARMY 


1910 


\  or  /-^ 


WASHINGTON 

GOVERNMENT  PRINTING  OFFICE 

1910 


I  ^  I 


A 


WAR   DEPARTMENT, 

Document  No.  3G6. 

Office  of  the  Chief  Signal  Officer. 


War  Department, 
Office  of  the  Chief  of  Staff, 

Washington,  April  W,  1910. 
The  following  Manual  of  Visual  Signaling,  prepared 
in  the  Office  of  the  Chief  Signal  Officer,  is  approved 
and  herewith  published  for  the  information  and  guid- 
ance of  the  Regular  Army  and  the  Organized  Militia 
of  the  United  States,  and  supersedes  all  other  pam- 
phlets or  similar  instructions  heretofore  issued  upon 
the  subject.  Officers  and  men  of  the  Signal  Corps  will 
thoroughly  familiarize  themselves  with  the  instructions 
and  suggestions  contained  herein. 
By  order  of  the  Secretary  of  War. 

Tasker  H.  Bliss, 
Brig.  General^  General  Staff, 

Acting  Chief  of  Staff'. 

(3) 


211280 


TABLE  OF  CONTENTS. 


Paga 

Chapter  I . — Introduction 9 

Chapter  II. —  Visual  signaling  equipment. 

The  wand 11 

The  flag  kit: 

The  2-foot  flag  kit 12 

The  4-foot  flag  kit 12 

Care  of  flag  material 13 

Powers  and  limitations  of  flag  signaling 13 

The  heliograph: 

Historical 14 

Description 14 

Assembling 17 

Adjustment 20 

Operation 21 

Care  of  apparatus .^.  22 

Powers  and  limitations  of  the  heliograph 22 

The  signal  lantern: 

Acetylene 23 

Calcium  carbide 23 

Method  of  gas  generation 24 

Description 25 

Operation  and  care 30 

Powers  and  limitations  of  the  signal  lantern 35 

Rockets  and  shells: 

Description 35 

Operation 38 

Employment 40 

The  semaphore:  Description 40 

(5) 


Page, 

The  searchlight:  Methods  of  employment 41 

The  Coston  signals 41 

Very's  night  signals 42 

The  Ardois  system  of  signaling , 42 

Sound  signals 44 

Improvised  signal  methods 44 

Chapter  III. — Alphabets  or  systems  of  signals. 

Signal  alphabets: 

American  Morse 45 

Continental  Morse 45 

Army  and  navy 45 

Abbreviations 46 

Code  calls 47 

Execution  of  signal  alphabets. 47 

The  army  and  navy  alphabet 47 

The  Morse  alphabets 49 

International  code  of  signals: 

Description ,  -  -  -  51 

Two-arm  semaphore 51 

The  Ardois  system 52 

Coston  signals 54 

Vary's  ijight  signals 54 

Rocket  signaling 55 

Two-arm  semaphore  alphabet,  U.S.  Navy 57 

Summary  of  signals,  army  and  navy  alphabet 60 

Chapter  IV. — The  field  message. 

Definition ^4 

The  blank  form 64 

Writing  the  message 66 

Instructions  to  operators : 

Use  of  message  blank 66 

Duties  of  sending  operators 66 

Order  of  transmission 66 

Duties  of  receiving  operators 67 

Communications  confidential 67 

Checking  the  message 67 


Chapter  V. —  The  signal  station. 

Page. 

Location  of  stations: 

General  considerations 68 

Backgrounds 70 

Azimuth  of  stations 71 

Altitude 71 

Determination  of  background  color 72 

Choice  of  apparatus 73 

Miscellaneous  considerations 73 

Intervisibility  table 74 

Finding  a  station 75 

Operation  of  stations: 

Personnel 76 

Calls  and  personal  signals 78 

Opening  communication 79 

Commencing  the  message 80 

Sending  and  receiving 80 

Breaking 80 

Discontinuance  of  transmission 81 

Acknowledgment  of  receipt 81 

Station  records 81 

Formation  of  signals 82 

Repeating  the  message 83 

Signal  practice 83 

Chapter  VI . — Codes  and  ciphers. 

Codes  in  use 84 

Employment  of  codes 84 

Cipher  code 85 

The  War  Department  Code 86 

Cipher  code  in  field  work 87 

Field  ciphers: 

Description  and  use 87 

Forms  of  field  cipher 88 

Inversions 88 

Concealment  of  terminations 88 

Cipher  apparatus:  The  cipher  disk 89 

The  mathematical  cipher 93 

The  route  cipher 94 

Cipher  detection:  Employment  of  cipher  disk 96 


Chapter  VII. — Field  glasses  and  telescopes. 

Pagft 

Reflection 98 

Refraction : 98 

Lenses 98 

Focus 99 

Optical  center 99 

Image 99 

Conjugate  foci 99 

Law  of  foci 1 100 

Formation  of  image 101 

Spherical  aberration 102 

Chromatic  aberration 102 

Telescopes ' 104 

Galilean  field  glasses  and  telescopes 106 

Porro  prism  field  glasses  and  telescopes 106 

Field  glasses 108 

Properties  of  telescopes  and  field  glasses : 109 

Power 109 

Light Ill 

Field 1 14 

Definition 115 

Field  glasses  and  telescopes  issued  by  the  Signal  Corps 119 

Type  A .^ 121 

Type  B 124 

Type  C 125 

Type  D 125 

Field-glass  specifications 126 


Chapter  I. 

INTRODUCTION. 

While,  in  consequence  of  the  development  of  elec- 
trical invention  and  improvement,  visual  signaling 
will  be  less  frequently  resorted  to  in  future  than  here- 
tofore in  the  service  of  field  lines  of  information,  it 
should  be  appreciated  that  the  necessity  for  an  ade- 
quate supply  of  apparatus  of  this  kind,  and  the  need 
for  skilled  manipulators  to  operate  it,  has  in  no  wise 
diminished.  The  great  celerity  with  which  electric 
signals  can  be  exchanged  and  their  usual  entire  inde- 
pendence of  local  conditions  has  placed  systems  of 
this  class  foremost  among  the  signaling  methods  of 
the  world.  There  is  scarcely  any  commercial  industry 
whose  successful  existence  does  not  vitally  depend 
upon  some  one,  perhaps  several  systems  of  signaling, 
and  improvements  of  old  and  inventions  of  new  signal 
devices  are  continually  necessary  to  meet  the  requisite 
needs  demanded  by  the  progress  of  art  and  science. 
Railways  are  probably  the  greatest  of  all  commercial 
users  of  signals.  With  them  the  great  mass  of  intelli- 
gence is  transmitted  by  the  electric  telegraph  and  tele- 
phone, but  the  flag,  the  semaphore,  the  signal  light, 
and  many  other  contrivances  furnish  indispensable 
visual  adjuncts.  Visual  signaling  is  and  always  will 
be  a  most  valuable  means  of  transmitting  information 

(9) 


10 

in  peace  and  war,  and  it  is  not  to  be  imagined  that 
it  will  ever  be  supplanted  in  its  particular  function  by 
the  introduction  of  other  methods.  Occasions  will 
frequently  occur  in  the  field  when  no  other  means 
will  be  practicable,  and  then,  if  not  before,  will  the 
value  of  the  system  be  fully  emphasized. 

Strictly  speaking,  a  visual  signal  is  any  visible  sign 
by  which  intelligence  is  communicated,  but  in  a  mili- 
tary sense  the  term  visual  signaling  has  a  broader 
meaning  and  includes  other  methods  of  transmitting 
information  than  those  which  appeal  to  the  sense  of 
sight. 

In  most  systems  of  signals  suitable  for  military  use, 
each  signal  is  composed  of  one  or  more  separate  units, 
known  as  elements.  Having  prescribed  a  certain 
number  of  elements,  the  various  signals  are  formed  by 
having  these  elements  appear  singly  or  together  in 
different  arrangements  or  combinations.  The  conti- 
nental system  is  one  of  two  elements,  namely  the  dot 
and  the  dash,  while  the  Morse  system  employs  three 
elements,  the  dot,  the  dash,  and  the  space.  Having 
agreed  upon  a  certain  number  of  combinations  of 
elements,  a  system  of  signals  is  formed  by  giving  a 
meaning  to  each  combination.  These  meanings  usu- 
ally include  the  letters  of  the  alphabet  and  numerals, 
combinations  of  which  being  used  to  formulate  neces- 
sary information.  Combinations  of  elements  of  any 
system  can  also,  however,  be  used  to  indicate  any 
desired  meaning. 

With  reference  to  period  of  visibility,  signals  are  of 
two  kinds,  transient  and  permanent.  A  transient 
signal  is  one  which  disappears  as  soon  as  completed; 


11 

a  permanent  signal  is  one  that  remains  in  view  for 
some  time.  Heliograph  signals  are  transient  signals, 
while  signals  made  by  code  flags  are  permanent  sig- 
nals. Signals  are  divided  into  classes  in  accordance 
with  the  number  of  elements  employed  in  their  forma- 
tion. Thus,  signals  using  two  elements  are  signals  of 
the  second  class,  signals  using  three  elements  signals 
of  the  third  class,  etc. 

The  standard  apparatus  used  in  .visual  signaling  is 
fully  described  in  a  succeeding  chapter.  Some  of  the 
instruments  employed  are  used  wholly  for  day,  and 
some  wholly  for  night,  signaling.  Some  devices,  either 
with  or  without  slight  variations,  are  equally  well 
adapted  to  day  or  night  work.  Visual  signaling  pre- 
sents a  great  field  for  ingenious  and  resourceful  work, 
and  emergency  will  often  demand  the  advantageous 
employment  of  other  methods  than  those  described 
herein. 


Chapter  II. 
VISUAL  SIGNALING  EQUIPMENT. 
THE    WAND. 

The  wand  is  a  stick  of  light  wood  about  18  inches 
long  and  one-half  inch  in  diameter.  It  is  held  loosely 
between  the  thumb  and  forefinger  and  waved  rapidly 
to  the  right  or  left  to  indicate  the  elements  of  the 
alphabet.  It  is  used  for  practice  purposes  and  the 
signals  made  by  it  are  only  intended  to  be  read  at 
very  short  distances. 


12 

THE    FLAG    KIT. 

Two  kinds  of  flag  kits,  the  2-foot  kit  and  the  4-foot 
kit,  are  issued  by  the  Signal  Corps. 

The  2-foot  Jcit. — This  kit  consists  of  one  white  and 
one  red  signal  flag,  two  three-jointed  staffs,  and  a 
suitable  carrying  case  to  contain  the  outfit.  The 
white  flag  is  made  of  white  muslin  2  feet  square,  with 
an  8-inch  turkey-red  muslin  center.  The  red  flag  is 
of  similar  size  and  material,  the  only  difference  being 
an  alternation  of  colors  in  the  body  and  center.  The 
means  of  attachment  to  the  staff  consists  of  a  loop 
at  the  center,  and  two  ends  of  white  tape  at  each  edge, 
of  the  back  of  the  flag  body.  The  staff  is  made  of 
hickory  in  three  joints,  each  23  inches  long,  and  is 
assembled  by  telescoping  into  brass  ferrules.  Brass 
eyes  are  provided  on  the  first  and  second  joints  to 
receive  the  tape  ends  at  the  edge  of  the  flag.  The 
carrying  case,  of  convenient  size  and  shape  to  con- 
tain the  two  flags  and  staffs  complete,  is  made  of 
8-ounce  standard  khaki  bound  with  leather  and  fitted 
with  a  shoulder  strap. 

The  2-foot  kit  is  essentially  a  practice  kit,  although 
under  favorable  conditions  of  weather  and  terrain  it 
may  be  used  to  advantage  as  a  short  distance  service 
signaling  outfit.  Two  of  these  kits  are  issued  to  each 
troop,  battery,  and  company  for  the  purpose  of  dis- 
seminating general  instruction  in  military  signaling 
throughout  the  army. 

The  4-foot  Tcit. — This  kit  is  of  essentially  the  same 
description  as  the  2-foot  kit  except  as  regards  size. 
The  flags  are  3  feet  9  inches  square  with  12-inch  cen- 


13 

ters  and  the  staffs  are  considerably  heavier,  the  joints 
being  each  36  inches  long.  The  4-foot  kit  is  the  stand- 
ard field  flag  kit  and  the  range  at  which  signals  can  be 
exchanged  with  it  depends  on  a  variety  of  factors, 
such  as  the  condition  of  the  weather,  the  location  of 
stations,  the  proficiency  of  signalmen,  etc.  The  speed 
for  continuous  signaling  is  seldom  greater  than  five 
to  six  words  per  minute. 

Care  of  flag  material. — Signal  flags  should  be  ex- 
amined at  the  close  of  drill  or  practice  and  repairs 
made  to  any  rents  or  loose  ties  discovered.  Flags, 
when  soiled,  should  be  thoroughly  washed  and  dried 
in  the  sun.  Signals  made  by  clean  flags  are  much  more 
easily  read  than  those  made  by  dirty  ones.  Staffs 
should  be  handled  with  care,  especially  when  jointing 
or  unjointing.  Care  should  be  taken  not  to  bruise 
the  ends  of  the  brass  ferrules.  If  a  ferrule  becomes 
loose  on  a  staff  it  should  be  tightened  without  delay. 

Powers  and  limitations  of  flag  signaling. — The  ad- 
vantages which  may  be  claimed  for  this  method  of 
signaling  are  portability  of  apparatus,  adaptability 
to  varied  weather  conditions,  and  great  rapidity  of 
station  establishment.  The  disadvantages  are  the 
lack  of  celerity  of  the  signals,  their  impenetrability 
to  dust  or  smoke,  and  the  comparatively  short  ranges 
at  which  they  can  be  read. 

THE   HELIOGRAPH. 

The  heliograph  is  an  instrument  designed  for  the 
purpose  of  transmitting  signals  by  means  of  the  sun's 
rays. 


14 

Historical. — Experiments  with  the  heHograph  with 
a  view  to  its  adoption  as  a  part  of  the  visual  signahng 
equipment  of  the  United  States  Army  were  com- 
menced as  early  as  1878.  The  reported  successful  use 
of  the  instrument  by  the  British  in  India  about  this 
time  led  to  the  importation  of  two  heliographs  of  the 
Mance  pattern.  A  series  of  experiments  with  these 
machines  conducted  for  the  purpose  of  eliminating 
certain  objectionable  features  finally  resulted  in  the 
evolution  of  the  present  type  of  service  heliograph. 

The  early  English  heliograph  was  not  provided  with 
a  shutter,  the  flash  being  directed  on  the  distant  station 
by  means  of  a  movable  mirror  controlled  by  a  key. 
The  great  objection  to  this  type  of  instrument  was  the 
impossibility  of  maintaining  accurate  adjustment  dur- 
ing the  transmission  of  signals  due  to  the  fact  that  the 
manipulation  of  the  mirror  tended  to  throw  the  flash 
constantly  out  of  alignment.  To  overcome  this,  the 
American  heliograph  has  been  provided  with  a  screen 
designed  to  operate  as  a  shutter  and  control  the  flash 
reflected  from  an  immobile  mirror. 

Description. — The  service  heliograph  equipment  of 
the  Signal  Corps  consists  of: 

A  sole-leather  pouch  with  shoulder  strap  containing — 

1  sun  mirror.       1  ..     ,       ,  .  -,      -, 

-,    .  ^.  .         \  Inclosed  in  a  wooden  box. 

1  station  mirror.  J 

1  screen,  1  sighting  rod,  1  screw-driver. 

A  small  pouch,  sliding  by  2  loops  upon  the  strap  of  the  larger 

pouch,  containing  1  mirror  bar. 

A  skeleton  leather  case  containing  2  tripods. 

The  mirrors  are  each  4i-inch  squares  of  plate  glass 
supported  by  sheet  brass  and  cardboard  backings,  and 
mounted  in  brass  retaining  frames.     At  the  center  of 


15 

each  mirror  there  is  an  unsilvered  spot  three  thirty- 
seconds  of  an  inch  in  diameter  and  holes  corresponding 


Fig.  1.— Heliograph  assembled. 


to  these  spots  are  drilled  in  the  backing.     The  sun 
mirror  differs  from  the  station  mirror  only  in  that  it 


16 


has  a  paper  disk  pasted  upon  its  face  covering  the 
unsilvered  spot.  The  mirror  frames  are  carried  by 
brass  supports  provided  at  the  bases  with  conical  pro- 
jections accurately  turned  to  fit  the  sockets  of  the  mir- 
ror bar  and  grooved  at  the  ends  to  receive  the  clamping 
spring.  Each  support  is  fitted 
with  a  tangent  screw  and  worm 
wheel  attachment  functioned  to 
control  the  motion  of  the  mirror 
frame  about  its  horizontal  axis. 

The  mirror  bar  is  a  bronze 
casting  provided  at  the  center 
with  .a  clamp  threaded  to  fit  the 
screw  of  the  tripod.  By  releas- 
ing the  clamp  the  bar  may  be 
moved  independently  of  the  screw 
and  adjusted  to  any  desired  posi- 
tion. Conical  sockets  for  the  re- 
ception of  the  mirror  supports  are 
provided  at  the  ends  of  the  mirror 
bar.  These  sockets  work  freely  in 
the  bar  and,  being  actuated  by  a 
tangent  screw  and  worm  wheel, 
serve  to  regulate  the  motion  of 
the  mirror  frame  about  its  ver- 
tical axis.  Clamp  springs,  for 
engaging  and  securing  the  ends 
of  the  mirror  frame  supports,  are  attached  at  each 
end  of  the  bar. 

The  screen  is  a  brass  frame  6i  inches  square,  in 
which  six  segments  or  leaves  are  mounted  in  such  a 
way  as  to  form  a  shutter.     The  leaves  are  designed  to 


Fig.  2.— Mirror  and  mirror 
bar  case. 


17 


turn  through  arcs  of  90°  on  horizontal  axes,  unanimity 

of  movement  being  secured  by  connections  made  with 

a  common  crank  bar.     The  crank  bar 

is  operated  by  a  key  and  retractile 

spring  which  serve  to  reveal  and  cut 

off  the  flash.     A  set  screw  and  check 

nut  at  the  lower  edge  of  the  screen 

frame  limits  the  motion  of  the  crank 

bar  and  the   opening  of   the  leaves. 

A  threaded   base    support   furnishes 

the   means    of    attaching  the  screen 

frame  to  the  tripod. 

The  sighting  rod  is  a  brass  rod  6^ 
inches  long,  carrying  at  the  upper 
end  a  front  sight  and  a  movable 
disk.  About  the  rod  is  fitted  a  mov- 
able bronze  collar,  coned  and  grooved 
to  take  the  socket  and  clamping 
spring  of  the  mirror  bar.  A  milled 
edged  bronze  washer  serves  to  clamp 
the  collar  to  the  rod  at  any  desired 
point. 

The  tripods  are  similar  in  all  re- 
spects, the  screw  of  either  threading 
into  the  mirror  bar  or  screen  frame. 
Each  tripod  is  provided  with  a  hook 
at  the  base  of  the  head,  allowing  the 
suspension  of  a  weight  when  great 
stability  is  required. 

Assembling. — There  are  two  ways 
of  assembling  the  heliograph  and  the  position  of  the 
sun  is   the  guide  in  determining  which  of   the  two 

40422—10 2 


Fig.  3.— Heliograph 
tripods. 


18 

should,  in  any  given  case,  be  employed.  When  the 
sun  is  in  front  of  the  operator  (that  is,  in  front  of  a 
plane  through  his  position  at  right  angles  to  the  line 
joining  the  stations)  the  sun  mirror  only  is  required; 
with  the  sun  in  rear  of  this  plane  both  mirrors  should 
be  used.  With  one  mirror  the  rays  of  the  sun  are 
reflected  directly  from  the  sun  mirror  to  the  distant 
station;  with  two  mirrors,  the  rays  are  reflected  from 
the  sun  mirror  to  the  station  mirror,  and  thence  to 
the  distant  station. 

With  one  mirror:  Firmly  set  one  of  the  tripods 
upon  the  ground;  attach  the  mirror  bar  to  the  tripod; 
insert  and  clamp  in  the  sockets  the  sun  mirror  and 
sighting  rod,  the  latter  having  the  disk  turned  down. 
At  a  distance  of  about  6  inches,  sight  through  the 
center  of  the  imsilvered  spot  in  the  mirror  and  turn 
the  mirror  bar,  raising  or  lowering  the  sighting  rod 
until  the  center  of  the  mirror,  the  extreme  point  of 
the  sighting  rod,  and  the  distant  station  are  accurately 
in  line.  Firmly  clamp  the  mirror  bar  to  the  tripod, 
taking  care  not  to  disturb  the  alignment,  and  turn  up 
the  disk  of  the  sighting  rod.  The  mirror  is  then 
moved  by  means  of  the  tangent  screws  until  the 
'^shadow  spot^^  falls  upon  the  paper  disk  in  the  sight- 
ing rod,  after  which  the  flash  will  be  visible  at  the 
distant  station.  The  ^'shadow  spot''  is  readily  found 
by  holding  a  sheet  of  paper  or  the  hand  about  6  inches 
in  front  of  the  mirror,  and  should  be  constantly  kept 
in  view  until  located  upon  the  disk.  The  screen  is 
attached  to  a  tripod  and  established  close  to,  and  in 
front  of,  the  sighting  disk,  in  such  a  way  as  to  inter- 
cept the  flash. 


19 

With  two  mirrors:  Firmly  set  one  of  the  tripods  on 
the  ground;  clamp  the  mirror  bar  diagonally  across 
the  line  of  vision  to  the  distant  station;  clamp  the 
sun  mirror  facing  the  sun  to  one  end  of  the  mirror 
bar  and  the  station  mirror  facing  the  distant  station. 
Stooping  down,  the  head  near  and  in  rear  of  the  sta- 
tion mirror,  turn  the  sun  mirror  by  means  of  its  tan- 
gent screws  until  the  whole  of  the  station  mirror  is 
seen  reflected  in  the  sun  mirror  and  the  unsilvered 
spot  and  the  reflection  of  the  paper  disk  accurately 
cover  each  other.  Still  looking  into  the  sun  mirror, 
adjust  the  station  mirror  by  means  of  the  tangent 
screws  until  the  reflection  of  the  distant  station  is 
brought  exactly  in  line  with  the  top  of  the  reflection 
of  the  disk  and  the  top  of  the  unsilvered  spot  of  the 
sun  mirror;  after  this  the  station  mirror  must  not  be 
touched.  Now  step  behind  the  sun  mirror  and  adjust 
it  by  means  of  the  tangent  screws  so  that  the  ^^ shadow 
spot''  falls  upon  the  center  of  the  paper  disk  on  the 
station  mirror.  The  flash  will  then  be  visible  at  the 
distant  station.  The  screen  and  its  tripod  are  estab- 
lished as  described  in  the  single  mirror  assembling. 

Alternate  method  wiiJi  two  mirrors:  Clamp  the  mirror 
bar  diagonally  across  the  line  of  vision  to  the  distant 
station,  with  the  sun  mirror  and  the  station  mirror 
approximately  facing  the  sun  and  distant  station, 
respectively. 

Look  through  small  hole  in  sun  mirror  and  turn  the 
station  mirror  on  its  vertical  and  horizontal  axes  until 
the  paper  disk  on  the  station  mirror  accurately  covers 
the  distant  station. 


20 

Standino;  behind  sun  mirror,  turn  it  on  its  hori- 
zontal and  vertical  axes  by  means  of  the  tangent 
screw  attachments  until  the  shadow  spot  falls  upon 
the  paper  disk  on  station  mirror. 

Adjustment. — Perfect  adjustment  is  maintained  only 
by  keepinpj  the  ^^ shadow  spot"  uninterruptedly  in 
the  center  of  the  paper  disk,  and  as  this  ^^spot"  con- 
tinually changes  its  position  with  the  apparent  move- 
ment of  the  sun,  one  signalman  should  be  in  constant 
attendance  on  the  tangent  screws  of  the  sun  mirror. 
Movement  imparted  by  these  screws  to  the  mirror 
does  not  disturb  the  alignment,  as  its  center  (the  un- 
silvered  spot)  is  at  the  intersection  of  the  axes  of  rev- 
olution. Extra  care  bestowed  upon  preliminary  ad- 
jvistment  is  repaid  by  increased  brilliancy  of  flash. 
With  the  alignment  absolutely  assured  and  the 
^^ shadow  spot"  at  the  center  of  the  disk,  the  axis  of 
the  cone  of  reflected  rays  is  coincident  with  the  line 
of  sight  and  the  distant  station  receives  the  greatest 
intensity  of  light.  Remember  the  distant  observer 
is  unquestionably  the  better  judge  as  to  the  character 
of  the  flash  received;  and  if  therefore,  adjustment  is 
called  for  when  the  ^^ shadow  spot"  is  at  the  center  of 
the  disk,  the  alignment  is  probably  at  fault  and 
should  be  looked  after  at  once.  In  setting  up  the 
tripods  always  see  that  the  legs  have  a  sufficient 
spread  to  give  a  secure  base  and  on  yielding  soil  press 
firmly  into  the  ground.  Keep  the  head  of  the  tripod 
as  nearly  level  as  possible  and  in  high  wind  ballast  by 
hanging  a  substantial  weight  to  the  hook.  See  that 
the  screen  completely  obscures  the  flash;  also  that 
the  flash  passes  entire  when  the  screen  is  opened. 
This  feature  of  the  adjustment  is  partially  regulated 


21 

by  the  set  screw  attached  to  the  screen  frame.  The 
retractile  spring  should  sharply  return  all  the  leaves 
of  the  screen  to  their  normal  positions  when  the  key 
is  released.  Failure  to  respond  promptly  is  obviated 
by  strengthening  or  replacing  the  spring. 

Operation. — It  is  of  the  utmost  importance  that 
uniformity  in  mechanical  movement  of  the  screen  be 
cultivated,  as  lack  of  rhythm  in  the  signals  of  the 
sender  entails  '^breaks"  and  delay  on  the  part  of  the 
receiver.  Dark  backgrounds  should,  when  practi- 
cable, be  selected  for  heliograph  stations,  as  the  sig- 
nals can  be  most  easily  distinguished  against  them. 

To  find  a  distant  station,  its  position  being  unknown, 
reverse  the  catch  holding  the  station  mirror  and  with 
the  hand  turn  the  mirror  very  slowly  at  the  horizon 
over  the  full  azimuth  distance  in  which  the  distant 
station  may  possibly  lie.  This  should  be  repeated 
not  less  than  twice,  after  which,  within  a  reasonable 
time,  there  being  no  response,  the  mirror  will  be 
directed  upon  a  point  nearer  the  home  station  and 
the  same  process  repeated.  With  care  and  intelli- 
gence it  is  quite  probable  that,  a  station  being  within 
range  and  watching  for  signals  from  a  distant  station 
with  which  it  may  be  desired  to  exchange  messages, 
this  method  will  rarely  fail  to  find  the  sought-for 
station. 

The  exact  direction  of  either  station  searching  for 
the  other  being  unknown,  that  station  which  first 
perceives  that  it  is  being  called  will  adjust  its  flash 
upon  the  distant  station  to  enable  it  when  this  light 
is  observed  to  make  proper  adjustments.  If  the  posi- 
tion of  each  station  is  known  to  the  other,  the  station 


22 

first  ready  for  signaling  will  direct  a  steady  flash  upon 
the  distant  station  to  enable  the  latter  to  see  not  only 
that  the  first  station  is  ready  for  work,  but  to  enable 
the  distant  station  to  adjust  its  flash  upon  the  first 
station. 

Smoked  or  colored  glasses  are  issued  for  the  purpose 
of  relieving  the  strain  on  the  eyes  produced  by  read- 
ing heliograph  signals. 

Care  of  apparatus. — Minor  parts  of  the  instrument 
should  be  dismounted  only  to  effect  repairs,  for  which 
spare  parts  are  furnished  on  requisition.  Steel  parts 
should  be  kept  oiled  and  free  from  rust .  Tangent  screws 
and  bearings  should  be  frequently  inspected  for  dust 
or  grit.  Mirrors  should  invariably  be  wiped  clean 
before  using.  In  case  of  accident  to  the  sun  mirror, 
the  station  mirror  can  be  made  available  for  substitu- 
tion therefor  by  removing  the  paper  disk.  If  the  tri- 
pod legs  become  loose  at  the  head  joints,  tighten  the 
assembling  screws  with  the  screw-driver. 

Powers  and  limitations  of  the  heliograpJi. — ^Porta- 
bility, great  range,  comparative  rapidity  of  operation, 
and  the  invisibility  of  the  signals  except  to  observers 
located  approximately  on  a  right  line  joining  the  sta- 
tions between  which  communication  is  had,  are  some 
of  the  advantages  derived  from  using  the  heliograph 
in  visual  signaling. 

The  principal  disadvantage  results  from  the  entire 
dependence  of  the  instrument  upon  the  presence  of 
sunlight.  The  normal  working  range  of  the  helio- 
graph is  about  30  miles,  though  instances  of  its  having 
attained  ranges  many  times  greater  than  this  are  of 
record.  The  heliograph  can  be  depended  upon  to 
transmit  from  five  to  twelve  words  per  minute. 


23 

THE   ACETYLENE    LANTERN. 

The  signal  lantern  is  an  instrument  designed  for  the 
purpose  of  transmitting  signals  by  means  of  intermit- 
tent flashes  of  artificial  light.  It  is  the  standard  night 
visual  signaling  equipment  furnished  by  the  Signal 
Corps  and  depends  for  its  illumination  upon  the  com- 
bustion of  acetylene  gas. 

Acetylene. — Acetylene  is  a  pure  hydrocarbon  gas, 
producible  in  various  ways,  the  commoner  of  which 
are:  (a)  By  dropping  calcium  carbide  into  water;  (b) 
by  dropping  water  upon  calcium  carbide.  This  gas 
gives,  when  burning,  high  penetrative  power,  and  was 
first  described  by  Mr.  Edmund  Davy,  professor  of 
chemistry  to  the  Royal  Dublin  Society,  in  1836. 

Calcium  carbide. — In  the  manufacture  of  calcium 
carbide  for  commercial  purposes  the  best  quality  of 
coke  and  quicklime  are  used.  These  two  substances 
are  powdered  thoroughly,  mixed  in  proper  porportions, 
and  then  placed  in  an  electrical  furnace.  Under  the 
action  of  the  intense  heat  (5,500°  F.)  these  two  refrac- 
tory substances  unite  and  form  calcium  carbide.  Cal- 
cium carbide  is  of  a  grayish-white  color,  crystal  in 
appearance,  and  is  nonexplosive  and  noncombustible, 
being,  except  for  its  affinity  for  water,  an  absolutely 
inert  substance.  A  pound  of  commercial  carbide  will 
produce  approximately  5  cubic  feet  of  gas.  When 
water  is  brought  in  contact  with  calcium  carbide,  the 
generation  of  acetylene  is  rapid;  owing  to  its  strong 
affinity  for  water  it  will  become  air  slacked  and  slowly 
lose  its  strength  if  exposed  to  the  action  of  the  moisture 
in  the  atmosphere;  consequently,  when  stored  or 
being  transported  it  should  be  kept  in  air-tight  cans. 


24 

When  calcium  carbide  is  brought  in  contact  with 
water,  the  following  occurs: 

As  is  known,  the  principal  components  of  water  are 
oxygen  and  hydrogen,  and  calcium  carbide  is  calcium 
and  carbon.  When  brought  in  contact,  the  oxygen  in 
the  water  decomposes  the  calcium  in  the  carbide,  and 
in  this  decomposition  the  hydrogen  in  the  water  is 
liberated  and  unites  with  the  carbon  of  the  carbide, 
forming  a  hydrocarbon  gas  wliich  is  acetylene.  It  is 
a  pure  white  light  of  intense  brilliancy  and  high  candle- 
power.  The  spectrum  analysis  of  acetylene  shows  that 
it  is  almost  identical  with  sunlight,  and  in  consequence 
delicate  shades  of  color  appear  according  to  their  true 
value  as  under  the  light  of  the  sun,  consequently  it 
penetrates  fog  to  a  greater  distance  than  other  lights. 
Acetylene  is  like  other  gases — explosive  when  mixed 
with  air  in  proper  proportions,  confined,  and  ignited — 
and  the  same  precautions  should  therefore  be  taken  in 
its  use  as  would  be  in  the  handling  of  coal  or  water 
gas,  gasoline  vapor,  etc.  As  acetylene  is  very  rifli  in 
carbon,  it  will  not  burn  in  its  pure  state  without  smok- 
ing. To  avoid  this,  burners  have  been  constructed  so 
that  the  gas  is  mixed  with  the  proper  proportion  of  air 
at  the  burner  tip,  to  insure  perfect  combustion.  The 
burners  for  acetylene  are  different  from  those  for  other 
gases.  In  order  to  get  a  flat  flame,  the  gas  is  brought 
through  two  perfectly  round  holes  at  an  angle  which 
causes  the  two  flames  to  impinge  upon  each  other  and 
thus  form  a  flat  flame. 

Method  of  gas  generation. — The  method  employed 
for  producing  acetylene  in  the  signal  lantern  is  by 
bringing  water  into   contact   with   calcium   carbide. 


25 

The  disadvantage  of  this  method  is  that  when  the 
water  is  not  in  excess  and  does  not  entirely  surround 
and  touch  each  piece  of  carbide  the  heat  of  generation 
will  so  change  the  chemical  properties  of  the  gas  that 
combustion  at  the  burners  is  not  satisfactory. 

This  change  is  technically  known  as  '^polymeriza- 
tion/' or  the  breaking  up  of  acetylene  into  other 
hydrocarbons,  such  as  vapors  of  benzine,  benzole,  etc. 
These  form  a  tarry  substance  which  is  apt  to  condense 
at  the  burner  tip  and  clog  the  openings.  Also  they 
deposit  carbon  on  the  burners,  as  they  require  more 
air  for  perfect  combustion  than  does  pure  acetylene. 
Another  disadvantage  of  this  system  is  that  after  the 
carbide  and  water  are  in  contact,  generation  of  gas 
will  continue  until  all  the  water  is  absorbed.  Where, 
however,  portability  of  the  generating  apparatus  is 
desired  and  resort  to  this  method  is  necessary,  the 
objections  are  not  important,  if  the  apparatus  is  well 
constructed  and  care  is  taken  in  its  use. 

Description. — This  equipment  consists  of  a  signal 
lantern  with  cartridge  generator  attached.  The  lan- 
tern is  equipped  with  a  special  aplanatic  lens  mirror, 
5  inches  in  diameter  and  about  3  inches  focus.  The 
lantern  is  packed  complete  in  a  wooden  case  with 
shoulder  straps  and  the  following  extra  parts  are 
included,  each  part  having  its  own  receptacle  in  the 
case :  2  burners ;  1  cover  glass ;  3  cartridges  of  calcium 
carbide  of  5  ounces  each;  1  pair  of  gas  pliers;  1  tube 
white  lead;  1  extra  filter  bag;  1  screw-driver. 

The  lantern  is  made  of  brass,  all  parts  of  which  are 
riveted.  The  burner  is  of  the  double  tip  form,  con- 
suming three-quarters  of  a  cubic  foot  per  hour.     The 


26 


lantern  is  fitted  with  a  hood  to  provide  proper  venti- 
lation and  at  the  same  time  to  prevent  the  flickering 
of  the  light  by  the  wind.  The  front  door  of  the  lan- 
tern is  hinged  and 
fastens  with  a 
spring  clasp;  it  is 
so  arranged  that  it 
can  be  entirely  re- 
moved if  necessary. 
The  cover  glass  is 
made  in  three  sec- 
tions and  is  not 
affected  by  the  ex- 
pansion and  con- 
traction of  the 
metal  due  to 
changes  in  temper- 
ature. The  glass  is 
fastened  by  the  aid 
of  a  spring  wire^ 
so  that  it  can  be 
readily  removed  if 
it  is  necessary  to 
replace  a  broken 
section.  In  the 
base  of  the  lantern 
is  a  key  and  the 
adj  ustment  for  reg- 
ulating the  height 

FIG.  4.-The  signal  lantern.  ^f      ^^^      flame. 

The  key  is  so  arranged  that  when  not  depressed  but 
little  gas  is  admitted  through  the  by-pass  to  the  burner 


27 


and  the  flame  is  low.  By  depressing  the  key  as  much 
gas  as  can  be  entirely  consumed  is  admitted  to  the 
burner,  which  gives  a  bright  flash.  At  the  back  of 
the  lantern  there  is  an  adjustable  handle,  so  that  the 
equipment  can  be  used  as  a  hand  lantern  if  desired. 
This  form  of  lantern  can  be  used  with  the  regular 
heliograph  tripod,  the  generator  being  either  attached 
to  the  back  of  the  lantern  or  suspended,  as  shown  in 
figure  4.  When  practicable  it  is  better  to  attach  the 
generator  to  the  lantern, 
as  shown  in  figure  5.  The 
candlepower  of  this  lan- 
tern is  about  1,900. 

TTie  generator  used  is 
known  as  ''the  cartridge 
generator,'^  and  while 
constructed  on  the  water- 
feed  principle,  the  disad- 
vantages incident  to  this 
method  are  eliminated  as 
far  as  possible.  It  is  con- 
structed of  brass  and  has 
a  removable  top.  At- 
tached to   the   inside   of 

the  top  is  a  flexible  frame  with  a  spring  latch,  the 
spring  latch  being  hinged.  (Fig.  8.)  At  the  top  of 
the  frame  is  a  tube  or  cylinder,  the  bottom  of  which 
is  conical  in  shape  and  covered  by  a  rubber  plug.  At 
the  bottom  of  the  frame  is  a  hollow  tube,  which  is  the 
water  inlet.  The  cartridge  proper  consists  of  a  tin 
cylinder,  having  an  opening  at  either  end.  A  small 
cylinder  of  wire  mesh  extends  from  and  connects  these 


Fig.  5. 


28. 

openings.  The  carbide  lays  around  this  mesh  on  the 
inside  of  the  cartridge.  The  rubber  plug  before  men- 
tioned fits  into  the  upper  opening,  and  the  water  tube 
into  the  lower  opening.  (See  figs.  7,  8,  and  9.)  Inside 
the  tube,  at  the  top  of  the  frame,  is  a  filter,  the  func- 
tion of  which  is  to  remove  the  dust  and  moisture  from 
the  gas.  The  outlet  from  this  chamber  is  by  a  brass 
bent  tube  having  a  stopcock  attached  thereto. 

Figure  6  gives  a  sectional  view  of  the  generator  with 
the  cartridge  in  place.  D  F  G  H  represent  the  valve 
frame  and  /  the  cartridge  attached.  The  reservoir  A 
is  filled  with  water,  and  when  the  frame  is  immersed, 
with  the  valve  R  closed,  the  air  contained  in  the  car- 
tridge and  tubing  can  not  escape,  the  water  seal  pre- 
venting, while  the  confined  air  prevents  the  water 
from  rising  in  the  tube  N.  When  the  valve  at  R  is 
opened  and  the  air  is  allowed  to  escape,  part  of  the 
water  from  the  reservoir  rises  into  the  tube  N  and  then 
out  through  the  small  hole  0  to  the  carbide.  Gas  is 
immediately  generated,  the  pressure  of  w^hich  prevents 
further  ingress  of  the  water  from  the  tube  iV,  and  the 
generation  of  gas  is  suspended. 

As  the  gas  passes  out  through  the  valve  at  R  the 
pressure  decreases,  permitting  the  water  to  again  rise 
in  the  tube  and  flow  through  0,  Gas  is  again  gener- 
ated, which  at  once  exerts  its  pressure  and  cuts  off 
the  supply  of  water.  This  is  the  automatic  action 
by  which  water  is  brought  in  contact  with  the  calcium 
carbide.  Thus  it  will  be  observed  that  the  use  or 
escape  of  the  gas  regulates  the  generation  by  the  sim- 
ple device  of  the  rise  and  fall  of  a  water  column. 
There  is  a  cap  M  screwed  over  the  tube  N.     This  is 


29 


used  to  deflect  the  course  of  the  water  downward,  so 
that  the  carbide  in  the  lower  part  of  the  cartridge  is 
first  attacked.  There  is  a  needle  inside  of  cap  M, 
which  can  be  used  for  cleaning  the  hole  0.  When  the 
gas  is  generated  it  passes  through  the  filter  D  on  its 
way  to  the  burner  through  R.  This  filter  consists  of  a 
tube  loosely  packed  with  ordinary  nonabsorbent  cot- 
ton, which  should  never  cover  the  escape  pipe  leading 
to  the  valve  R.  In 
passing  through  this 
cotton  filter  mois- 
ture and  dust  are  re- 
moved from  the  gas. 
In  the  latest  model 
a  felt  filter  is  used 
instead  of  cotton. 

The  escape  pipe  F 
provides  a  means  for 
the  escape  of  gas  gen- 
erated and  not  used 
or  generated  more 
rapidly  than  con- 
sumed. Should  an 
excess  be  generated, 
it  passes  down 
through  the  tube  F,  and,  finding  its  way  through  some 
small  holes  in  the  bottom  of  this  tube,  escapes  through 
the  water  seal  and  the  opening  at  G.  It  will  be 
noted  that  if  escaping  gas  at  G  should  become  acci- 
dentally lighted,  the  flame  can  not  strike  back  into 
the  filter  and  cartridge  because  of  the  water  seal.  The 


Fig.  6.— Signal  lantern  generator. 


30 

principal  things  to  observe  in  the  operation  of  this 
generator  are  the  following: 

(1)  To  see  that  the  rubber  plugs  ft  tightly  into  the 
openings  of  the  cartridge. 

(2)  That  the  tube  N,  the  cap  M,  and  water  hole  0 
are  not  stopped  up. 

(3)  That  the  cotton  in  the  filter  is  changed  fre- 
quently. 

(4)  That  the  stopcocTc  R  is  closed  before  inserting  tJie 
frame  in  the  water.  If  this  latter  instruction  is  not 
complied  with,  it  can  be  readily  seen  that  the  watei* 
will  have  free  access  to  the  carbide  and  excessive 
generation  will  occur. 

When  the  charge  is  exhausted,  the  entire  cartridge 
is  taken  out  and  thrown  away.  This  eliminates  the 
handling  of  carbide  and  the  disagreeable  task  of  clean- 
ing out  the  residuum  after  the  gas  has  been  extracted. 

Connection  is  made  from  the  stopcock  R  to  the  hose 
connection  on  the  lantern  proper,  and  this  is  the  pas- 
sageway of  the  gas  from  the  generator  to  the  burner. 
As  soon  as  the  stopcock  is  opened  the  water  rises 
through  the  tube  and  flows  to  the  carbide.  The 
advantage  of  the  cartridge  being  submerged  in  the 
water  is  to  reduce  and  absorb  as  much  of  the  heat 
liberated  by  generation  as  is  possible.  These  lanterns 
have  been  tested  up  to  a  distance  of  10  miles  with  the 
naked  eye,  and  under  favorable  conditions  can  be  used 
over  a  range  somewhat  in  excess  of  this.  With  a 
30-power  telescope  the  flash  can  be  read  at  a  distance 
of  30  miles. 

Operation  and  care. — Take  the  lamp  and  generator 
from  the  case  by  aid  of  the  handle  attached  to  the 


31 

lamp;  screw  the  complete  outfit  on  a  heliograph 
tripod,  or  stand  the  outfit  on  a  level  object;  remove 
the  cover  of  generator,  to  which  is  attached  the  flexi- 
ble frame  (fig.  9);  detach  spring  from  the  catch  of 
the  flexible  frame;  tear  off  flaps  from  the  ends  of 
carbide  cartridge  (or  pry  off  small  caps)  and  attach 
the  cartridge  as  shown  in  figure  9.  Then  attach  to 
frame  as  shown  in  figure  10,  being  careful  to  see  that 
both  rubber  plugs  fit  tightly  into  the  holes  in  the 
cartridge ;  fasten  the  latch  of  the  spring  over  the  metal 
catch;  close  stopcock  R  on  service  pipe;  completely 
fill  the  outer  can  of  generator  with  water,  the  object 
being  to  have  the  generator  level  full  of  water  when 
the  lamp  is  in  service,  then  immerse  the  frame  and 
cartridge,  pressing  the  top  of  the  generator  down 
tight.  In  doing  this  the  water  will  overflow  the  sides 
of  the  generator  tank.  Now  connect  by  rubber  tubing 
the  stopcock  with  the  gas  inlet  at  the  bottom  of  the 
lamps,  as  shown  in  figure  4;  then  (1)  open  front  door 
of  the  lamp,  (2)  light  a  match,  (3)  open  stopcock,  and 
(4)  light  the  gas  at  the  burner.  In  doing  this  hold 
the  key  open.  In  the  new  model  the  key  and  hose 
connection  are  on  the  side  of  bottom  of  lamp. 

When  the  gas  is  ignited,  the  lamp  is  ready  for  sig- 
naling, and  the  key  can  be  operated  as  is  the  Morse 
telegraph  instrument,  but  of  course  not  so  rapidly. 

In  the  event  of  the  flame  being  too  high  when  the 
key  is  closed,  adjustment  can  be  made  by  loosening 
the  set  screw  (fig.  4,  indicated  by  an  arrow)  and  adjust- 
ing the  light  by  turning  screw  &.  When  at  the  proper 
height,  tighten  the  set  screw  which  locks  the  by-pass 
in  its  proper  position.     In   the  new  model   this  is 


32 


accomplished  by  aid  of  the  regulator  by-pass  valve  at 
the  left-hand  side  of  bottom  of  lamp.  The  lamp  is 
properly  adjusted  when  shipped  and  should  not  be 
changed  unless  absolutely  necessary.  Connect  the 
rubber  tube  to  the  burner  before  opening  the  stopcock 
on  the  generator. 

To  recharge  the  generator,  take  the  frame  and  the 
old  cartridge  from  the  case,  throw  away  the  old  case 
and  replace  with  a  fresh  one,  proceeding  as  before. 

See  that  fresh  water 
is  put  in  the  genera- 
tor each  time  a  new 
cartridge  is  used. 

In  the  tube 
through  which  the 
service  pipe  passes  is 
a  felt  filter  for  taking 
the  dust  out  of  the 
gas.  If  the  filter 
clogs,  unscrew  the 
cap  to  which  the 
service  pipe  is  at- 
tached, clean  the 
felt,  or  replace  it 
with  a  new  filter,  binding  it  in  place  by  a  stout  thread 
or  string. 

If  the  burner  of  the  lamp  does  not  produce  a  per- 
fectly flat  flame  it  has  become  clogged  and  should  be 
cleaned  with  the  burner  cleaner  furnished,  or  a  new 
burner  should  be  substituted,  care  being  taken  to  put 
a  little  white  lead  on  the  nipple,  if  practicable,  so  as  to 
insure  a  tight  joint. 


Fig.  7. 


33 


In  repacking  the  outfit  in  the  case,  throw  out  the 
water  and  wipe  the  can  and  generator  parts  dry.  You 
can  not  be  too  careful  to 
keep  the  apparatus  clean. 
This  is  especially  true  of 
the  small  pipe  that  passes 
up  through  the  bottom  of 
the  cartridge,  with  a  cap 
over  it.  The  cap  should 
always  be  screwed  in 
place,  as  its  object  is  to 
prevent  the  water  from 
squirting  to  the  top  of 
the  cartridge. 

The  back  of  the  lamp 
can  be  removed  by  turn- 
ing the  small  thumbscrew^ 
on  the  top  and  drawing  out  the  pin  which  holds  the 
shell  into  which  is  fitted  the  lens.     It  is  not  necessary 


Fig.  I 


Fig.  9. 


to  take  the  back  out  except  to  replace  a  kns,  as  the  lat- 
ter can  be  cleaned  by  opening  the  front  door. 
40422—10 3 


34 

If  it  is  desirable  to  use  the  lamp  as  a  hand  lantern 
the  flame  can  be  turned  on  full  by  turning  the  button 
in  a  vertical  position;  this  locks  the  key  open.  In 
the  new  model  depress  the  key  and  lock  it  with  the 
latch  above  the  key. 

One  charge  of  calcium  carbide  will  supply  gas  to 
burn  about  one  hour  with  the  light  turned  on  full,  or 
for  approximately  three  hours^  signaling. 


Fig.  10. 


If  signaling  is  to  be  suspended  for  some  hours,  empty 
the  water  out  of  the  generator  and  close  valve  R. 

The  glass  front  can  be  replaced  by  taking  out  the 
wire  spring.  The  glass  cuts  should  be  mounted  in  a 
horizontal  position  and,  to  prevent  breaking,  should 
be  protected  from  rain  when  the  lamp  is  hot.  If  a 
glass  should  be  broken  and  an  extra  one  is  not  avail- 
able to  replace  it,  signaling  can  be  continued  by  turn- 
ing the  flame  on  full  and  using  the  heliograph  shutter, 


35 

a  cap  or  piece  of  board  in  front  of  the  lantern  to 
obscure  and  reveal  the  flash.  Without  the  protection 
of  the  cover  the  flame  is  easily  blown  out  when  turned 
low,  but  will  not  be  extinguished  even  in  a  strong 
wind  if  the  gas  is  turned  full  on. 

Old  model  lamps  are  serially  numbered  from  1  to 
200,  inclusive;  the  new  model  lamps  are  serially  num- 
bered from  201  upward. 

Powers  and  limitations  of  the  acetylene  signal  lan- 
tern.— As  conditions  are  usually  more  uniform  at  night 
than  in  the  daytime,  the  signal  lantern  is  probably  the 
most  reliable  of  all  visual  signaling  outfits.  The 
advantages  of  this  form  of  apparatus  are  its  porta- 
bility, speed  of  operation,  and  comparatively  great 
range.  The  principal  disadvantages  are  due  to  the 
interference  caused  by  rain,  fog,  and  moonlight.  The 
speed  attainable  with  the  lantern  is  about  the  same  as 
that  attainable  with  the  heliograph. 

ROCKETS    AND    SHELLS. 

Two  distinct  kinds  of  rockets  and  shells  are  issued, 
one  of  which  is  adapted  to  day  and  the  other  to  night 
signaling.  Shells  and  rockets  of  the  amber  smoke 
type  with  parachutes  are  used  in  the  daytime,  while 
shells  (red  and  white)  and  sequence  rockets  are  used 
at  night. 

Description. — The  shells  are  all  single  shot  and  are 
fired  from  a  5-inch  portable  mortar,  attaining  a  height 
of  about  550  feet.  The  report  of  explosion  can  be 
heard  at  varying  distances  up  to  5  miles,  depending 
on  weather  conditions.  The  parachute  attached  to 
the  smoke  shell  suspends  a  small  light  wooden  tube 


36 


Fig.  11.— Signaling  rocket  and  accessories. 


37 


Fig.  12.— Signaling  shells. 


38 

which,  after  ignition,  emits  smoke  for  from  four  to 
six  seconds.  The  red  and  white  shells,  on  bursting, 
discharge  a  shower  of  red  and  white  fire  which  can 
be  observed  for  some  time,  in  fact  almost  until  the 
sparks  fall  to  the  ground. 

Rockets  for  both  day  and  night  signaling  are 
equipped  with  parachutes.  The  smoke  rocket  is  of 
similar  construction  to  the  smoke  shell.  The  sequence 
rocket  is  so  arranged  at  the  base  that  threaded  sections 
of  combustible  material  burning  either  red  or  white 
can  be  attached  to  it.  Rockets  ascend  about  700 
feet. 

Each  rocket  and  shell  is  supplied  in  a  cylindrical 
sealed  tin  can,  which  also  contains  a  port  fire,  wind 
matches,  and  for  the  rockets  a  stick  in  four  sections. 
On  the  outside  of  the  can  is  a  label  designating  the 
kind  of  shell  or  rocket  therein  contained.  These  cans 
are  easily  opened  by  pulling  a  ring  and  require  no 
special  opening  tool. 

Operation. — In  firing  shells  the  mortar  should  be 
surrounded  by  earth  or  sand,  preferably  placed  in 
sacks.  The  fuse  for  all  shells  is  very  rapid  and  should 
be  ignited  by  attaching  the  port  fire  to  a  long  stick. 

All  of  the  old  type  Signal  Corps  mortars,  originally 
designed  to  withstand  a  pressure  of  1,000  pounds  per 
square  inch,  and  made  of  ordinary  iron  pipe,  are  con- 
sidered unsafe  and  should  be  immediately  destroyed. 
The  new  mortars,  recently  made  for  the  Signal  Corps 
by  the  Ordnance  Department,  are  of  cold-drawn  steel 
having  a  tensile  strength  of  6,000  pounds  per  square 
inch,  which  is  more  than  the  maximum  pressure  for 
firing    any   of   the   Signal   Corps   bombs.     They    are 


39 

stamped  ^^  Signal  Corps,  U.  S.  A.,  Model  1907/Vor 
'^Rocket  Gun,  Watertown  Arsenal,  1907.'^ 

The  sequence  rocket  is  prepared  for  use  by  attach- 
ing red  or  white  sections  to  the  base  in  such  a  combina- 
tion as  to  form  letters  of  the  alphabet  which  it  is  de- 
sired to  use.  Letters  containing  the  same  color  in 
sequence  are  very  difficult  to  read  and  should  be 
avoided  whenever  possible.  If  necessary  to  use  them, 
blank  sections  furnished  for  the  purpose  should  be 
inserted  between  the  units.  The  base  of  the  rocket 
will  secure  six  units. 

When  rockets  are  to  be  fired  the  sticks  must  be 
firmly  attached,  the  rocket  placed  upright  in  a  trough, 
upon  a  frame,  or  against  a  post.  If  the  fuse  is  be- 
neath the  paper  covering  the  ^^  choke '^  orifice,  the  paper 
should  be  torn  off  and  the  rocket  ignited  by  a  port 
fire.  In  the  rockets  now  used  the  fuse  extends 
through  the  covering  and  can  be  lighted  direct.  If 
the  night  be  damp  this  fuse  should  be  exposed  only  a 
moment  before  the  rocket  is  fired.  If  several  rockets 
are  to  be  fired  in  succession  it  is  well  to  prepare  them 
all  at  the  same  time,  and  to  have  them  all  stood  up- 
right, but  each  separated  from  the  other  at  a  dis- 
tance of  at  least  6  feet,  else  one  may  ignite  the  other 
accidentally.  In  firing  for  chronosemic  signals,  one 
rocket  ought  to  be  kept  ready  upon  the  frame  and  in 
reserve,  to  be  fired  in  place  of  one  that  fails. 

If  a  rocket  misses  fire  it  is  to  be  taken  from  the  stand 
and  laid  on  the  ground .  Its  place  is  at  once  supplied 
by  a  similar  rocket,  fired  in  its  stead.  The  failing 
rocket  is  laid  on  the  ground  pointed  away  from  the 
station  in  order   that  if  it  has  only  hung  fire   and 


40 

should  afterwards  ignite  it  may  not  disarrange  the 
signal  shown  or  injure  any  one  of  the  party.  If  the 
wind  blows  freshly  the  rocket  to  be  fired  should  be 
inclined  slightly  against  the  wind. 

Signal  rockets  and  shells  are  furnished  in  sealed 
cans  and  should  not  be  removed  therefrom  until 
ready  for  use.  Strict  economy  should  be  observed  in 
the  use  of  these  articles  and  on  no  account  should  they 
be  used  for  purposes  of  display. 

Employment. — Rockets  and  shells  are  especially 
valuable  in  making  preconcerted  or  emergency  signals. 
On  account  of  the  great  amount  of  ammunition  re- 
quired it  is  impracticable  to  spell  out  messages  with 
them.  These  articles  should  be  supplied  to  outposts, 
detached  stations,  etc.,  to  be  used  for  signaling  the 
approach  of  the  enemy  or  the  happening  of  unex- 
pected events,  the  necessity  for  promptly  knowing 
which  is  important. 

THE    SEMAPHORE. 

If  signal  stations  are  to  be  permanently  occupied,  and 
it  is  impracticable  to  electrically  connect  them,  com- 
munication may  be  facilitated  by  erecting  semaphores. 

Semaphores,  while  primarily  used  for  day  signaling, 
can  be  advantageously  used  at  night  by  attaching 
lights  to  the  arms. 

The  navy  semaphore  consists  of  four  arms  pivoted 
at  the  ends,  three  on  one  side  of  the  upright,  or  pole, 
and  one  on  the  other  side.  These  arms  have  three 
positions:  Horizontal;  upward  at  an  angle  of  45°  to 
the  horizontal;  downward  at  an  angle  of  45°  to  the 
horizontal. 


41 

Full  instructions  for  the  operation  of  the  semaphore, 
and  also  for  the  use  of  balls,  cones,  drums,  pennants, 
and  whef  ts  as  distant  signals,  are  given  in  the  Interna- 
tional Code  of  Signals. 

THE    SEARCHLIGHT. 

The  electric  searchlight,  when  available,  can  often 
be  successfully  employed  for  night  signaling,  frequently 
affording  efficient  means  of  communication  between 
ships  and  shore  stations,  when  wireless  working  is 
impracticable.  This  system  of  visual  signaling  is 
practicable  and  especially  valuable  where  the  stations 
are,  on  account  of  the  terrain,  not  intervisible. 

Methods  of  employment. — In  signaling  with  the 
searchlight  the  usual  method  of  handling  the  shaft 
or  beam  is  identical  with  that  employed  with  the  flag. 
In  the  first  position  the  beam  is  shown  vertically, 
while  motions  to  the  right,  the  left,  and  directly 
serve  to  indicate  the  elements  of  the  alphabet.  Chro- 
nosemic  signals  may  also  be  used  in  searchlight  sig- 
naling, the  shaft  of  light  being  directed  intermittently 
on  some  conspicuous  object,  such  as  a  cloud,  balloon, 
or  high  mountain  top. 

COSTON    SIGNALS. 

These  signals  are  pyrotechnic  compositions  which 
burn  with  great  intensity  of  light  and  color.  The 
colors  red,  white,  and  green  are  found  best  suited 
for  signaling.  The  signals  are  prepared  in  the  form 
of  cartridges  and  are  burned  from  a  holder.  The 
colors  burned  may  indicate  the  elements  of  any 
alphabet,  or  such  other  special  signals  as  may  be 
desired. 


42 
very's  night  signals. 

The  Very  system  employs  projected  red,  white,  and 
green  stars,  which  are  shot  from  pistols  held  in  the 
hand. 

Description. — The  Very  pistol  is  a  breechloading, 
single-shot  pistol  with  an  8-inch  steel  barrel  cham- 
bered to  receive  a  12-gauge  commercial  shotgun 
shell.  Brass  shells  are  used  and  are  packed  in  boxes 
colored  to  indicate  the  character  of  stars  employed 
in  loading.  The  color  of  the  star  fired  may  indicate 
an  element  of  any  alphabet  or  any  special  signal  which 
may  be  desired.  The  stars  rise  to  a  height  of  about 
200  feet  and  remain  visible  for  some  time. 

THE    ARDOIS    SYSTEM. 

The  Ardois  system  is  a  special  system  of  night  sig- 
naling designed  to  utilize  combinations  of  red  and 
white  signal  lights  in  forming  the  elements  of  any 
desired  alphabet.  Four  signal  lamps  capable  of  dis- 
playing either  red  or  white  lights  are  attached  at 
convenient  intervals  to  a  vertical  cable  or  staff  rigged 
between  the  top  of  a  mast  and  the  deck,  if  on  ship- 
board, or  the  ground,  if  on  shore.  Illumination  is 
furnished  by  electrical  means  and  any  desired  combina- 
tion of  lights  is  automatically  obtainable  by  operating 
a  keyboard. 

This  system  is  valuable  on  vessels  or  at  permanent 
shore  stations,  but  the  great  expense  of  installation 
precludes  its  general  use.  Wiring  diagrams  and 
technical  instructions  relative  to  this  apparatus  are 
in  all  cases  furnished  when  the  same  is  issued. 


43 


Fig.  13.— The  Very  pistol. 


44 


SOUND   SIGNALS. 


When  recourse  to  any  method  of  sight  signals  can 
not  be  had  on  account  of  weather  conditions  or  lack 
of  suitable  apparatus,  sound  signals  may  often  be 
advantageously  used.  The  commoner  means  of  fur- 
nishing sound  signals  are  the  horn  and  the  whistle, 
though  many  other  kinds  of  apparatus  are  practi- 
cable. The  necessary  elements  of  any  system  can 
be  indicated  by  one  short,  two  shorts,  and  a  long 
blast.  The  advantage  of  this  system  of  signaling  is 
that  it  can  be  used  in  any  kind  of  weather,  both  in  day- 
time and  at  night.  On  the  other  hand,  sound  signals 
are  generally  more  difficult  to  read  than  sight  signals 
and  tend  to  disclose  the  presence  of  stations  to  hostile 
forces. 

IMPROVISED    SIGNALING    METHODS. 

The  object  of  this  chapter  has  been  to  describe  only 
the  standard  visual  signaling  equipment  issued  and 
generally  utilized.  Besides  the  methods  detailed, 
there  are  many  others  which  may  be  successfully  em- 
ployed by  the  ingenious  signalman  when  the  necessity 
for  them  arise.  The  use  of  any  means  of  transmitting 
sigijals  whatever  is  justifiable  when  for  any  reason  the 
regular  apparatus  is  not  available.  Special  conven- 
tional scout  signals  are  given  in  paragraph  82,  Field 
Service  Regulations. 

In  the  field  many  instances  will  occur  where  it  will 
be  necessary  to  transmit  information  rapidly  without 
recourse  to  the  authorized  equipment.  This  will  be 
especially  true  of  outposts,  detached  stations,  patrols, 
and  other  small  bodies  of  troops,  and  it  will  devolve 


45 

upon  individual  commanders  to  improvise  methods 
of  signaling  best  suited  to  the  occasion  and  the  con- 
veniences at  hand. 


Chapter  III. 

ALPHABETS  OR  SYSTEMS  OF  SIGNALS. 

SIGNAL    ALPHABETS. 

a.m;erican  continental  atimy  and 

Kietters —  .  m^orse.  aiorse.  navy. 

A _  -  —  22 

B 2112 

C - 121 

D 222 

E -  12 

F _.  2221 

G 2211 

H 122 

I --  1 

J 1122 

K — -■_  2121 

L 221 

M 1221 

N _ .  —  _  11 

0 21 

P 1212 

Q __  1211 

R 211 

S : .--  2J2 

T _  —  2 

U _  112 

V 1222 

W 1121 

X 2122 

Y --  _-  111 

Z '.-  2222 

& 

tion 1112 


46 


Niiiiieral»- 

1 

2 

3 

4 

5 

6 

7 


9 

0 

Piinetuation— 

.    Period 

_  -  - 

:    Colon 

Ko 

;    Semicolon 

Si 

,    Comma 



P   Interrogation 



I    Exclamation 



Fraction  line 

_ 

-   Hyphen 

Hx 

'    Apostrophe 

£  Pound  Sterling . . . 

0   Parenthesis 

Pn 

"  Quotation  marks . . 

Qn 

Paragraph 



Brackets 

Bn 

Dollar  mark 

Sx 

Dash  . 

Dx 

Underline 

Ux 

AMERICAN 

CONTINENTAL 

ARMY   AND 

MORHE. 

MORSE. 

NAVY. 





nil 





2222 



-    -   -   —   __ 

1112 



__ 

2221 



1122 



2211 





1222 





2111 





1221 
.       2112 

The  following  abbreviations,  conventional  signals, 
and  code  calls  are  authorized  in  visual  signaling: 

ABBREVIATIONS. 


a after. 

b before. 

c can. 

h have. 

n not. 

r are. 


t the. 

u you. 

ur your. 

w word. 

wi with. 

y yes. 


47 

CODE  CALLS. 

International  Code  use ICU 

(Navy)  telegraph  dictionary  use TDU 

(Navy)  geographical  list  use GLU 

(Navy)  general  signal  use GSU 

Navy  list  use NLU 

Vessel's  numbers  use VNU 

Cipher ''A"  use« CAU 

Cipher  ''B "  use  ^ CBU 

Cipher ''C"  use  a CCU 

Although  the  use  of  but  one  alphabet  is  authorized 
in  visual  signaling  in  the  U.  S.  Army,  emergencies 
may  arise  where  it  may  be  imperative  to  use  either 
the  Army  and  Navy,  the  Continental  Morse,  or  the 
American  Morse  alphabet.  Instructions  for  the  use 
of  either  alphabet  under  such  conditions  are  given. 

EXECUTION  OF  SIGNAL  ALPHABETS. 

THE    ARMY    AND    NAVY    ALPHABET. 

SIGNALING     WITH     FLAG      OR     TORCH,     HAND     LANTERN,     BEAM     OF 
SEARCHLIGHT,   AND    HELIOGRAPH. 

There  is  one  position  and  three  motions.  The 
position  is  with  the  flag  or  other  appliance  held  ver- 
tically^ the  signalman  facing  directly  toward  the  sta- 
tion with  which  it  is  desired  to  communicate,  his  body 
erect  and  feet  sufficiently  separated  to  insure  stable 
equilibrium.  The  first  motion  C^one''  or  "1^^)  is  to 
the  right  of  the  sender,  and  will  embrace  an  arc  of 
90°,  starting  with  the  vertical  and  returning  to  it,  and 
will  be  made  in  a  plane  at  right  angles  to  the  line  con- 
necting the  two  stations.     The  second  motion  (^Hwo^' 

o  These  calls  are  for  preconcerted  use  in  or  with  the  navy. 


48 

or  ^^2^0  is  a  similar  motion  to  the  left  of  the  sender. 
The  third  motion  C^front/^  ^Hhree/^  or  ^^3^0  is 
downward  directly  in  front  of  the  sender  and  instantly 
returned  upward  to  the  first  position. 

The  beam  of  searchlight  will  be  ordinarily  used 
exactly  as  the  flag,  the  first  position  being  a  vertical 
one. 

To  use  the  torch  or  hand  lantern,  a  footlight  must 
be  used  as  a  point  of  reference  to  the  motion.  The 
lantern  is  more  conveniently  swung  out  upward  to 
the  right  of  the  footlight  for  "1/'  to  the  left  for  ^^2/' 
and  raised  vertically  for  ^^3.^' 

In  using  the  hehograph,  the  first  position  is  to 
turn  a  steady  flash  on  the  receiving  station.  The 
signals  are  made  by  short  and  long  flashes.  Use  short 
flashes  for  '^1/'  two  short  flashes  in  quick  succession 
for  "2/'  and  a  long,  steady  flash  for  ''3.''  The  ele- 
ments for  a  letter  should  be  slightly  longer  than  in 
sound  signals. 

Each  word;  abbreviation,  or  conventional  signal  is 
followed  by  ^^3.^' 

The  full  address  of  a  message  is  considered  as  one 
sentence  and  will  be  followed  by  the  signal  '^33.'' 

The  signal  to  indicate  that  '^ cipher  follows^'  and 
^^cipher  ends^'  is  with  the  flag  and  torch  '^XC3,"  and 
with  other  methods,  except  the  International  Code, 
by  ^^XC.^'  It  will  always  precede  and  follow  a  cipher 
message  or  such  part  of  a  plain  text  message  as  is 
enciphered. 

The  following  conventional  signals  are  authorized 
in  the  use  of  the  army  and  navy  alphabet: 


49 

End  of  a  word 3 

End  of  a  sentence 33 

End  of  a  message 333 

Numerals  follow  (or)  numerals  end xx3 

Signature  follows sig.  3 

Error 12  12  3 

Acknowledgment  (or)  I  understand 22  22  3 

Cease  signaling 22  22  22  333 

Cipher  follows  (or)  cipher  ends 2122  121  3 

Wait  a  moment 1111  3 

Repeat  after  (word) 121  121  3  22  3  (word) 

Repeat  last  word :  121  121  33 

Repeat  last  message 121  121  121  333 

Move  a  little  to  the  right 211  211  3 

Move  a  little  to  the  left 221  221  3 

Signal  faster 2212  3 

THE    MORSE    ALPHABETS. 

TO    SIGNAL   WITH  THE    FLAG,    TORCH,    HAND   LANTERN,    OR   BEAM   OF 
SEARCHLIGHT. 

The  dot  is  made  by  a  motion  to  the  right  of  the 
sender  embracing  an  arc  of  90°,  starting  from  the  ver- 
tical and  returning  to  it,  in  a  plane  at  right  angles  to 
the  line  connecting  the  two  stations. 

The  dash  is  made  by  a  similar  motion  to  the  left. 

The  space  which  occurs  only  between  dots  is  made 
by  prolonging  the  signal  for  the  last  dot  for  an  interval 
of  time  equal  to  the  time  of  an  additional  dot,  the 
staff  of  the  flag,  the  beam  of  the  searchlight,  etc., 
being  maintained  in  a  horizontal  position  for  the  time 
specified.  The  signal  so  made  would  therefore  repre- 
sent a  dot  and  space. 

The  letter  ^^C  is  accordingly  made  thus:  Right, 
right  prolonged,  right. 

40422—10 4 


50 

The  long  dash  C^L'')  is  distinguished  from  the  short 
dash  C^t'O  by  prolonging  the  signal  to  the  left  for  a 
period  of  time  equal  to  one  dot.  The  long  dash  rep- 
resenting ''naught'^  is  similarly  made  by  prolonging 
the  signal  to  the  left  for  a  period  of  time  equal  to  two 
dots. 

The  ''front"  signal  is  made  by  lowering  the  flag 
from  the  vertical  position  to  the  front  and  immediately 
returning  it  to  the  vertical  position. 

A  slight  pause  is  made  between  each  signal. 

The  following  conventional  signals  are  authorized, 
using  the  Morse  alphabets : 

End  of  word one  front. 

End  of  sentence two  fronts. 

End  of  message three  fronts. 

TO  SIGNAL  WITH  THE  HELIOGRAPH  OR  PLASH  LANTERN. 

The  dot  is  made  by  pressing  down  the  key  of  the 
shutter  and  immediately  releasing  the  same. 

The  short  dash  is  made  by  pressing  down  the  key 
and  holding  it  down  for  a  period  equal  to  two  dots. 

The  long  dash  (''L")  is  made  by  holding  down  the 
key  for  a  period  equal  to  three  dots  while  the  longer 
dash  (naught)  requires  the  key  to  be  held  down  for  a 
period  equal  to  four  dots. 

The  space  is  made  on  the  heliograph  as  in  ordinary 
telegraphy  by  the  absence  of  any  signals  for  a  period 
equal  to  the  time  of  one  dot. 

On  the  heliograph  the  letter  ''C"  is  made  as  follows: 
Short  flash,  short  flash,  interval,  short  flash. 

When  the  call  of  a  station  is  acknowledged,  both 
stations  will  adjust  e^ch  on  the  flash  of  the  other. 


FLAGS    AND    PENNANTS 

International  Code. 


Fig.  14-. 


p 

p  p 

P  -ps  p 

p 


"CODE  FLAG"  AND 
'ANSWERING  PENNANT." 


Fig.  15. 


51 

When  adjustments  are  satisfactory,  the  station  called 
will  acknowledge  and  cut  off  its  flash,  and  the  calling 
station  will  proceed  with  its  message. 

INTERNATIONAL    CODE    OF    SIGNALS. 

Description. — By  means  of  the  International  Code  of 
Signals  people  of  different  nationalities  may  communi- 
cate with  each  other,  although  neither  party  has  knowl- 
edge of  any  language  save  his  own  native  language. 
The  code  is,  as  its  name  indicates,  international,  and 
every  seagoing  vessel  of  every  nation  is  equipped 
with  its  flags.  The  Code  of  Signals  contemplates 
the  use  of  26  flags  (figs.  14  and  15) ;  one  for  each  letter 
of  the  alphabet  and  a  code  pennant.  Complete 
instructions  relative  to  the  use  of  this  code  are  con- 
tained in  a  book  issued  by  the  Hydrographic  Ofiice, 
Navy  Department,  and  known  as  the  ^^The  Interna- 
tional Code  of  Signals.^'  In  using  this  system  the  sig- 
nals are  displayed  by  hoisting  combinations  of  two, 
three,  or  four  flags.  All  possible  combinations  rep- 
resent words,  expressions,  or  phrases,  which  may  be 
found  in  the  ^^International  Code  of  Signals,"  referred 
to  above. 

Two-arm  semaphore. — This  system  is  frequently 
used  by  the  United  States  Navy,  the  folio  wing  instruc- 
tions covering  the  use  of  the  system: 

1.  To  communicate  with  a  station: 

Face  the  station  and  wave  the  flags  over  the  head 
to  attract  attention,  making  at  frequent  intervals  the 
call  letter  of  the  station.  When  the  station  called 
is  ready  to  receive  the  message,  it  answers  by  display- 
ing its  own  call  letter  until  the  sender  makes  the 


52 

^^alphabeticar^  or  '^numeral/'  as  the  case  maybe. 
Then  proceed  with  the  message.  At  the  end  of  each 
word  bring  the  flags  across  the  lower  part  of  the  body. 

2.  To  call  a  ship: 

Hoist  International  Code  letter  J  and  make  code  let- 
ter of  ship;  then  proceed  as  in  article  1. 

3.  To  make  a  general  semaphore  signal: 

Hoist  cornet ;  all  ships  answer  by  answering  pennant ; 
then  make  signal. 

4.  At  the  end  of  the  message  extend  the  arms  hori- 
zontally and  wave  the  flags  until  the  receiver  answers 
in  the  same  manner,  showing  that  the  message  is 
understood. 

Should  the  receiver  miss  a  word,  he  signifies  the  fact 
by  waving  the  flag  over  his  head.  The  sender  wifl 
then  cease  signaling  and  wave  his  flags  similarly  to 
show  that  he  understands.  The  receiver  then  makes 
''repeat  last  word/'  or  whatever  he  wishes  to  say. 

Should  the  sender  make  a  mistake,  he  will  make 
the  ''error"  signal  until  answered  by  the  receiver 
with  the  same  signal.  He  then  proceeds  with  the 
message. 

THE   ARDOIS    SYSTEM. 

In  using  this  system  in  connection  with  the  Arm}^ 
and  Navy  Code,  the  red  lamp  indicates  "1^^  and  the 
white  lamp  "2.'^  Four  lamps  are  placed  on  a  verti- 
cal staff  and  electrically  illuminated  to  indicate  the 
numerals  of  the  Myer  Code,  which  represents  the 
letters  of  the  alphabet.  For  instance,  white-white,  or 
"22,"  represents  the  letter  "A,"  and  white-red-red- 
white,  or  "2112,"  represents  the  letter  "B,"  etc.  In 
this  system  the  lights  indicating  the  letters  of  the 
alphabet  are  read  from  the  top  downward. 


53 


When  the  lamps  are  placed  horizontally,  they  are 
read  from  the  sender's  right  to  his  left,  and  conse- 
quently from  the  receiver's  left  to  his  right. 

When  the  letters  of  the  alphabet  are  to  be  used  to 
indicate  the  meaning  set  opposite  them  in  the  follow- 
ing tabulation,  the  upper  light  of  the  display  is  pul- 
sated. This  is  effected  by  means  of  a  special  pulsating 
key.  Special  signification  is  not  given  ^^I''  and  ^^T,'' 
they  being  represented  by  a  single  lamp. 


steady  display. 

Upper  light  pulsated. 

A 

Cipher  "A"  use. 

0  (naught). 

Repeat  (following  rule  for  conventional  signals  un 

wag  code). 
Telegraphic  dictionary  use. 
Error. 
4. 
6. 
Compass  signals  use. 

5. 

Negative. 

Geographical  list  use. 

9. 

Cipher '^B"  use. 

Cipher '^C"  use. 

Affirmative. 

Interrogatory. 

International  code  use. 

General  signals  use. 

Navy  list  use. 

Annulling. 

Numerals. 

Vessels'  number  use. 

2. 

3. 

8. 

Boat  signals  use. 

B                        .   .   . 

C 

ier  wig- 

D 

E     

F 

G 

H 

I 

J 

K 

L 

M 

N     

0 

P 

Q..;. 

R 

s 

T 

u 

V 

w 

X 

Y 

Z 

Letters 

Code  call 

Interval              

Before  numerals  are  made,  the  distinctive  signal  for 
^^ numerals"  ^^X"  is  shown  and  the  upper  light  is  pul- 
sated, which  serves  still  further  to  distinguish  them 
from  letters.  The  resumption  of  letters  after  using 
numerals  will  be  indicated  by  the  upper  light  being  no 


54 

longer  pulsated,  but  the  display  'betters''  C'3'')  will 
be  turned  on  as  an  additional  indication. 

The  acknowledgment  of  the  correct  receipt  of  a 
message  will  be  indicated  by  the  letter  '^K.''  If  the 
message  has  not  been  fully  received,  or  if  it  is  not 
understood,  indication  thereof  will  be  made  by  signal- 
ing the  letter  ^'G.'^ 

The  end  of  a  word  is  indicated  by  2212. 

COSTON    SIGNALS. 

Letters  of  the  army  and  navy  alphabet  may  be 
represented  at  night  by  Coston  lights,  port  fires,  or 
other  colored  pyrotechnical  lights  by  displaying  the 
^'red''  for  one  and  the  ^^ white '^  for  two. 

In  using  the  Morse  alphabet  the  ^^red^^  represents 
the  dot  and  the  ^' white '^  the  dash. 

Coston  signals  and  other  similar  lights  are  best 
suited  for  preconcerted  signals. 

VERY^S    NIGHT    SIGNALS. 

The  navy  signal  book  is  used,  to  which  the  follow- 
ing explanation  refers : 

The  letter  R  stands  for  red  and  the  letter  G  for 
green,  and  each  letter  designates  a  separate  star  or 
cartridge.  Bracketed  stars  are  a  pair  of  different 
colors,  discharged  together  from  two  pistols.  The 
system  is  based  on  the  Army  and  Navy  Code,  red  rep- 
resenting ''1'^  and  green  ^^2^ 

1— RRRR.  2— GGGG. 

3— RRRG.  4— GGGR. 

5— RRGG.  6— GGRR. 

7— RGGG.  8— GRRR. 

9_RGGR.  10— GRRRG. 


I   UNIVERSITY 

\  ^^  J 

^"^"-""''^  55 

Aflarmative,  or  "  Yes  ' RGRG 

Negative,  or  ''No".. .GRGR 

Numeral GRGG 

Interrogatory RGRR 

Annulling RRGR 

Divisional  point,  date,  designator,  or  interval GGRG 

Telegraphic  dictionary,  <^\  bracketed. 

Geographical  list,  <^\  followed  by  a  rocket. 

Boat  signals,  rocket  followed  by^  p>  '  , 

Navylist {1}    {1} 

General  call,  rocket  followed  by  G. 
Message  call,  G  without  the  rocket. 

The  squadron,  division,  section,  or  ship's  call,  the  ''number"  o: 
squadron,  division,  section,  or  ship. 

Answering,  or  "  I  understand  " R 

Repeating,  or  "  I  do  not  understand  " G 

Danger  or  distress,  R  repeated  several  times  in  quick  succession. 

ROCKET   SIGNALING. 

In  general,  rockets  and  shells  are  best  used  in  dis- 
playing preconcerted  signals. 

Sequence  rockets  may  also  be  used  to  display  differ- 
ent colored  lights  in  sequence  to  represent  letters  or 
numerals  of  the  army  and  navy  alphabet.  The 
method  of  attaching  the  sections  in  the  base  of  the 
sequence  rocket  is  described  in  Chapter  III.  In  using 
sequence  rockets  in  this  manner,  the  element  '^1'^  ol 
the  army  and  navy  alphabet  is  represented  by  a  red 
star,  while  a  white  star  represents  the  element  ^^2.^' 
To  send  the  letter  ^'A'^  a  rocket  showing  two  white 
stars  is  sent  up.  If  ''B^'  is  to  be  sent,  a  rocket  show- 
ing   white-red-red-white    is    discharged.     Each    star 


56 

bums  for  four  to  six  seconds,  and  there  is  a  slight 
interval  between  the  visibility  of  each  star.  Between 
two  or  more  stars  of  the  same  color,  as  ^^A/'  ^^N/' 
^^D/^  '^dummies/'  which  show  no  light  and  carry  the 
fire  to  the  next  star  to  be  ignited,  are  employed. 

In  the  preparation  of  codes  for  signals  with  rockets 
or  bombs  there  should  always  be  arranged  a  ^^prepara- 
tory signal''  which  means  '^Are  you  ready?"  etc.,  and 
an  ^'answering  signal,''  which  means  ^^ Repeat  your 
last  signal,"  etc.,  a  signal  ''annul,"  which  means  ''Dis- 
regard last  signal,"  and  a  signal  to  signify  the  correct 
receipt  of  the  complete  message,  or  "Signal  seen  and 
understood." 


57 


TWO-ARM  SEMAPHORE  ALPHABET,  U.  S.  NAVY. 


A     I 


B     2 


C    3 


\^ 


D    4 


E    5 


F    6 


G    7 


H    8 


I    9 


J      LETTERS 


K     0  (ZERO) 


58 


M 


N 


END  OF 
MESSAGE 


u 


/♦ 


w 


59 


ERROR  OR 
ATTENTION 


NUMERALS     END  OF  WORD     ANNULLING 


CONVENTIONAL  SIGNALS, 

End  of  word see  instruotioiiB. 

End  of  message see  instructions. 

Error ....see  instructions. 

Repeat  last  word C  '^nd  of  word',  onoe. 

Repeat  last  message C.  '©^d  of  word',  3  times. 

Us6  paper  and  pencil .  P,  'end  of  word',  twice. 

ABBREVIATIONS. 
A  "end  of  word"    after    T    "end  of  word'.,  the 
B    "    "     "     ...before  U       «    "     «     ..you 
C    "    "     "        can      UR    "    "     "     -your 
H    "    "     "       .have    W      "    "     "     -word 
N     "    "     "        not      Wl     "    "     "     -with 
R     -    "    "     ....are       Y        "    «     "     ..yes 

PG  "end  of  word," permission  granted 

NG   ^    "     "     permission  not  granted 

XX    "    "     "     .  ..numerals follow 


60 


SUMMARY  OF  SIGNALS,  ARMY  AND  NAVY  ALPHABET. 


CHARAC- 
TERS 


WIG 

WAG 

SYSTEM 


22 


Column 
4 


ELECTRIC 
NIGHT 
SYSTEM 


TWO—  ARM 
SEMAPHORE 


A 


HAND 
FLAGS 


Column 
6 


VERY'S 
SYSTEM 


Column 
7 


SECONDARY 
MEANINGS 


2112 


121 


222 


12 


2221 


\ 


O(ZERO) 


Repeat 


Error 


61 


Colon- 

2 


Column 
3 


Column 
4 


Column 
_5 


Column 

7 


2211 


H 


122 


1122 


p^^ 


K 


2121 


Negative 


221 


M 


1221 


/T 


j^ 


i 


N 


H 


62 


Colli  rr 
3 


1212 


Ow 


^ 


Column 

1 


Affirm- 
ative 


1211 


^R 

OW 

©R 


=/ 


•f 


Interrog- 
atory 


211 


Ow 

©R 


I. 


212 


Ow 

#R 

Ow 


TK 


Ow 


^: 


u 


112 


1222 


©R 
©R 

Ow 

OR 
Ow 
Ow 

Ow 


V 


rt"^ 


63 


Column 

I 


Column 
2 


Column 
3 


Cotui 


Column 
_5 


Column 
6 


Column  ' 
1 


W 


1121 


Ow 


Annulling 


2122 


Ow 


Ow 
Ow 


Numerals 


@R 

©R 


\ 


2222 


Ow 
Ow 
Ow 
Ow 


^r 


Cornet 


©R 


Letters 


112 


§>R 
0R 
Ow 


V 


General 

Signals 

Use 


2111 


Ow 

i)R 
<Sr 


Interval 


2212 


Ow 
Ow 

Cw 


inator 


64 

Chapter  IV. 

THE  FIELD  MESSAGE. 

Definition. — The  term  'Afield  message''  is  applied  to 
all  messages  sent  over  field  lines  of  information.  All 
field  messages  for  transmission  over  field  lines  of  infor- 
mation by  electrical  or  visual  means  should  be  plainly 
written  by  the  sender  on  the  blank  forms  in  the  United 
States  Army  Field  Message  Book.  The  practice  of 
verbally  delivering  telegrams  to  enlisted  men  for 
transmission  should  invariably  be  discouraged. 

^^In  framing  telegrams,  all  words  not  important  to 
the  sense  will  be  omitted.  The  last  name  of  the  officer 
addressed,  or  his  title,  and  the  last  name  of  the  sender 
are  generally  sufficient."  (Paragraph  1198,  Army 
Regulations.) 

The  lilanTc  form, — The  United  States  Army  Field 
Message  Book  issued  by  the  Signal  Corps  is  4f  inches 
wide  by  6}  inches  long,  and  contains  40  message  blanks 
with  duplicate  tissue  sheets  and  two  sheets  of  carbon 
paper. 

The  message  is  written  on  the  yellow  sheet,  which 
can  be  torn  out  for  delivery.  The  carbon  sheet  is 
attached  to  the  book,  and  contrary  to  the  custom  in 
most  carbon  duplicating  books,  is  placed  under  the 
tissue  sheet  when  a  message  is  being  written.  When 
not  being  used,  the  carbon  sheet  should  invariably  be 
kept  in  the  back  of  the  book.  When  the  upper  carbon 
sheet  has  become  worn  out,  it  should  be  torn  out  and 
the  second  carbon  sheet  used  instead.  The  blank 
form  is  shown  in  figure  16.  The  back  of  the  blank  is 
ruled  in  squares  and  provided  with  scales  for  use  in 
making  sketches. 


65 


O  -n  <  CD 
m  r-  5  c 

c 

^ 

2 

o 

:o  >  :33  N 
^     o  m  N 

C/) 

2. 

ER,    P 
LESS, 
,  CYCI 
MOUIN 

> 

a 

n 
o 

79 

E2  H  !-        3: 

B 

>^ 

8  m  ^  r  O 

B 

1   D  H  >  z 

1 

E,     TELE 
NTERN, 
FOOT  IV 
MESSENi 

:  means  used 

1 

p. 

r 

^ 

S 

^ 
J 

0, 

sipi 

§ 

f 

m  [I  Ti 

o 

;2 

?P  I 

w 

s 

p 

09 

r-fc 

ss 

t^ 



1 

g 

o 

3 

s 

^ 

9 

i 

.§ 

0 

>; 

1 
t 

*     Sent  by 
[These  s; 

V. 

i 

1  1 
1  § 
a  a 

Oo 

i 

o 

Time 

1.15 

A.  M. 

aces  for  £ 

Oq* 

Iw 

o 

II 

s 

1 

3 

1               ^ 

o        S 

tq    5' 

Orq 

"^         ^ 

CD 

jS 

(%          Q_ 

§^ 

O 

o 

w 

o 

^         B 

^ 

o 

o 

p 

^I  1 

i 

>r 

40422—10- 


66 

Writing  the  message. — In  writing  the  message  the 
name  of  the  sending  detachment  should  appear  after 
the  heading  ^'from^'  on  the  upper  Hne,  as  ^'from  Head- 
quarters 1st  Brigade/^  while  the  location  of  the  sender 
should  appear  on  the  second  line  after  the  heading 
^'at/'  The  heading  ^'hour^^  on  the  third  line  should 
show  the  hour  the  message  wsls  written  and  not  the  hour 
the  message  was  transmitted.  The  heading  ^'  received  ^' 
at  the  bottom  of  the  page  is  filled  in  by  the  addressee  and 
shows  the  time  of  the  receipt  of  the  message  by  him. 

INSTRUCTIONS  TO  OPERATORS. 

'Use  of  message  hlanlc, — The  field  message  blank  will 
be  used  for  field  messages  both  sent  and  received. 

Duties  of  sending  operators. — The  sending  operator 
will  enter  the  time  when  the  message  is  handed  him 
for  transmission  in  the  left-hand  corner  at  the  bottom 
of  the  blank  opposite  the  word  ^' Received.'^  He  will 
enter  in  the  proper  places,  at  the  head  of  the  blank,  the 
number  of  the  message,  the  call  letter  of  his  station, 
with  his  personal  signal,  the  check  (number  of  words 
or  groups  of  cipher  contained  in  message,  counting 
address  and  signature),  and,  after  ^'OK^'  has  been 
received,  he  will  enter  the  time  the  message  was  sent, 
and  the  call  letter  of  the  receiving  station,  with  the 
personal  signal  of  the  receiving  operator. 

Order  of  transmission. — To  transmit  a  message,  the 
operator  will  send:  (1)  The  number  of  message  and 
call  letter  of  his  station;  (2)  his  personal  signal;  (3) 
the  check;  (4)  ^^fm'^  followed  by  name  of  sending 
detachment;  (5)  '^  at''  followed  by  location  of  sending 
detachment  and  date;    (6)  ''Ho''  followed  by  hour 


67 

(a.  m.  or  p.  m.)  message  was  written;    (7)  address  in 

full;  (8)  period,  ( );  (9)  body  of  message;  (10) 

'^sig^'  (signature  follows) ;  (11)  signature. 

Duties  of  receiving  operators. — The  receiving  operator 
will  add  to  the  message  received,  the  month,  date,  and 
year,  and  omit  the  ''sig,^^  '^fm,^'  and  ^'at,''  and,  after 
satisfying  himself  that  the  check  and  number  of  words 
correspond  will  give  ^^OK'^  followed  by  the  call  letter 
of  his  station  and  his  own  personal  signal.  He  will 
then  enter  in  the  proper  places,  at  the  head  of  the 
blank,  the  call  letter  of  his  own  station,  with  his  per- 
sonal signal  and  the  time  the  message  was  received. 

Communications  confidential.  —  Communications 
transmitted  by  telegraph  or  signals  are  always  confi- 
dential and  will  only  be  revealed  to  those  officially 
entitled  to  receive  them. 

CTieclcing  the  message. — In  preparing  the  ^^ check" 
of  the  message,  all  words  and  figures  written  in  the 
address,  body  of  the  message,  and  the  signature  will 
be  counted. 

In  counting  the  check  of  a  message,  all  words, 
whether  in  plain  English,  code,  or  cipher,  pronounce- 
able or  unpronounceable,  or  initial  letters,  will  be 
counted  each  as  one  word.  The  abbreviations  for  the 
names  of  places,  cities,  towns,  villages.  States,  Terri- 
tories, and  Provinces,  will  be  counted  as  if  written  in 
full.  In  the  names  of  towns,  counties,  countries,  or 
States,  all  of  the  words  will  be  counted. 

Abbreviations  of  weights  and  measures  in  common 
use,  and  cardinal  points  of  the  compass,  will  be 
counted  each  as  one  word. 

To  prevent  liability  to  error,  numbers  and  amounts 
should  be  written  in  words,  and  when  not  so  written, 


68 

the  receiving  operator  will  request  that  it  be  done.  If 
the  writer  decUnes  to  write  the  amounts  in  words,  the 
message  will  be  accepted  as  written,  and  each  figure 
will  be  counted  as  one  word. 

Figures,  decimal  points,  and  bars  of  division,  and 
letters  will  be  counted  each  separately  as  one  word. 

In  ordinal  numbers,  the  affixes,  st,  d,  nd,  rd,  and 
th,  will  each  be  counted  as  one  word. 

In  transmitting  the  telegram  shown  in  figure  16,  the 
following  would  be  sent  by  the  operator : 

No  1  K  Mo  CK  14  OB  fm  Headquarters  let  Corps  at 
Taylor's  School  House  Kan  1  ho  1245  PM  to  Signals 
Platte  City  Mo.  Request  ten  miles  buzzer  wire  be 
sent  here  quick  sig  Jones 

Chapter  V. 

THE   SIGNAL    STATION. 
LOCATION    OF    STATIONS. 

In  field  operations  tactical  considerations  will 
usually  prescribe  within  certain  limits  the  number 
and  general  location  of  signal  stations.  The  general 
directions  for  deployment  being  given,  the  signalman 
will  be  called  upon  to  demonstrate  his  skill  in  the 
selection  of  particular  locations  most  conducive  to 
the  efficient  service  of  information. 

General  considerations. — Considering  all  things,  the 
best  location  for  a  signal  station  is  one  which  affords 
maximum  visibility  and  at  the  same  time  minimum 
exposure  to  hostile  observation.  These  conditions, 
apparently  paradoxical,  can  be  more  or  less  recon- 
ciled by  the  exercise  of  ingenuity  on  the  part  of  the 
signalist.  A  good  theoretical  knowledge  of  the  special 
requisites  of  signal  sites,  together  with  the  ability  to 


69 


70 

apply  it  to  the  conditions  arising  in  any  given  case, 
will  result  in  securing  the  best  obtainable  locations. 

The  first  essential  of  the  signal  station  is  visibility, 
the  second  being  that  of  concealment  from  hostile 
observation.  In  acquiring  a  mean  between  conflict- 
ing requirements,  the  following  special  considerations 
in  the  selection  of  stations  should  be  considered. 

Backgrounds. — Backgrounds  are  important  factors 
in  the  selection  of  signaling  sites. 

Sky  backgrounds  are  desirable  as  affording  strong 
contrast  and  are  therefore  conducive  to  celerity  in  the 
transmission  of  signals.  They  are  rare  and  can  only 
be  secured  when  stations  are  located  on  the  exact  crest 
of  ridges,  on  mountain  peaks,  or  on  lands  which  bound 
the  horizon  of  view  from  the  other  stations.  Stations 
with  sky  backgrounds,  while  affording  the  best  facil- 
ities for  transmission,  are  little  adapted  to  the  require- 
ment of  secrecy. 

Dark  backgrounds  are  far  more  common  and  more 
easily  obtainable  than  sky  exposures.  They  afford 
the  maximum  means  of  concealment  from  hostile 
observation,  but  materially  reduce  the  range,  speed, 
and  accuracy  of  signal  transmission. 

Mixed  or  broken  backgrounds  are  those  which  dis- 
play varied  colors  behind  the  signals.  Backgrounds 
of  this  description  do  not  accord  with  either  of  the 
essential  requirements  of  the  signal  station  and  should 
be  avoided  whenever  possible. 

In  general,  sky  backgrounds  should  always  be 
selected  for  signal  stations  when  conditions  are  such 
that  the  requirement  of  secrecy  can  be  dispensed  with; 
if,  on  the  other  hand,  there  is  reason  to  fear  that  the 
signals  may  be  intercepted  by  the  enemy,  dark  back- 


71 

grounds  should  invariably  be  chosen,  even  though  the 
disadvantages  they  impose,  render  them  less  desirable 
visually. 

Azimuth  of  stations. — ^The  azimuth  of  signal  stations 
should,  if  possible,  be  such  that  the  visual  lines  of 
information  should  intersect  the  vertical  plane  through 
the  apparent  course  of  the  sun,  at  a  considerable 
angle.  Stations  located  so  as  to  be  unavoidably 
viewed  from  these  directions  during  portions  of  the 
day  are  very  liable  to  appear  enveloped  in  a  haze,  and 
telescopes,  if  turned  upon  them,  are  filled  with  dazzling 
light.  If  the  location  of  stations  on  or  close  to  the 
sun  line  is  unavoidable,  sites  affording  sky  exposures 
should  be  chosen.  Exposures  of  this  kind  obviate  to 
a  great  extent  the  difficulty  of  sun  haze  and  should  be 
secured  when  this  difficulty  is  encountered  and  it  is 
impracticable  to  change  the  azimuth  of  the  station. 

Altitude. — The  location  of  signal  stations  at  high 
altitudes  will  tend  to  obviate  difficulties  arising  from 
smoke,  haze,  and  dust.  The  undulation  of  the  atmos- 
phere noticeable  on  a  hot  summer's  day  is  always  less 
at  a  distance  from  the  earth's  surface,  and  it  is  often 
practicable  to  read  signals  from  a  tree  or  housetop 
when  they  would  be  unintelligible  from  the  ground. 
This  air  undulation  is  less  over  spots  well  shaded  than 
those  exposed  to  the  glare  of  the  sun,  a  fact  that  should 
be  borne  in  mind  in  all  telescopic  examinations. 
Another  reason  for  locating  stations  at  high  altitudes 
is  because  the  cool  night  air,  the  smoke  and  dust  of  the 
day,  and  heavy  mists  lie  close  to  the  ground,  filling  the 
depressions  and  lowlands,  while  the  higher  points 
remain  in  view.  Stations  on  high  ground  are  then 
equally  well   adapted   to   day   and   night   signaling. 


72 

Sites  and  selections  of  this  kind  of  terrain  will  not  only 
oft^n  preclude  the  necessity  for  changes  of  location, 
but  also  will  allow  the  continuous  working  of  the 
station  when  signals  made  from  lower  positions  would 
be  invisible.  In  foggy  or  murky  weather  peaks  and 
mountain  tops  are  usually  enveloped  in  mist,  and 
under  these  conditions  stations  should  be  situated  on 
lower  ground. 

Determination  of  hacTcground  color. — The  color  of  the 
background  of  a  station  is  that  color  against  which  the 
signals  appear  to  be  displayed  when  viewed  from  the 
distant  station.  Having  chosen  a  point  entirely  in 
view  of  the  station  or  stations  to  be  communicated 
with,  and  having  fixed  the  exact  position  of  the  signal- 
ing apparatus,  the  color  of  the  background  should  be 
determined  as  carefully  as  conditions  of  terrain  will 
permit.  If  the  elevation  of  the  distant  station  is 
without  doubt  greater  than  that  of  the  home  station 
it  is  safe  to  assume  that  the  color  of  the  background 
will  be  that  of  the  objects  directly  around  and  behind 
it.  On  the  other  hand,  if  the  distant  station  unques- 
tionably occupies  the  lower  position,  a  sky  exposure 
will  usually  result.  In  locating  stations  it  is  very 
difficult,  if  not  impossible,  especially  at  long  ranges,  to 
determine  the  color  of  the  background  as  viewed  from 
the  distant  station  when  the  stations  are  approxi- 
mately on  the  same  level.  This  can  only  be  done  by 
proceeding  in  front  of  the  home  station  and  taking 
such  a  position  that  it  can  be  viewed  with  the  eye  on 
the  line  of  sight  between  the  stations.  The  telescope 
should  be  established  over  the  initial  point  of  the  home 
stations  and  directed  on  the  distant  station.  The 
observer  for  background  should  proceed  to  a  point 


73 

where  his  head  is  in  the  center  of  the  field  of  the  tele- 
scope. Looking  back  at  the  home  station  from  this 
point,  the  color  of  the  objects  about  and  just  behind 
the  initial  point  will  be  the  color  of  the  background. 
The  correct  deterinination  of  background  color  from 
the  vicinity  of  home  stations  is  usually  difficult  and 
unsatisfactory,  and  it  is  considered  the  best  method  to 
establish  communication  with  the  distant  station  by 
simultaneously  using  several  kinds  of  signaling  appa- 
ratus, that  kind  producing  the  most  intelligible  signals 
being  retained  fol*  continued  use. 

Choice  of  apparatus. — Sunlight  conditions  permit- 
ting, the  heliograph  will  ordinarily  be  used  for  day 
signaling  on  account  of  the  advantages  of  the  great 
range  and  speed  afforded  by  it.  When  its  use  is  pro- 
hibited by  weather  conditions,  the  flag  will  be  sub- 
stituted for  it.  The  white  flag  will  be  used  against 
dark  and  the  red  against  sky  or  broken  backgrounds. 
The  distant  station  is  the  better  judge  as  to  which 
color  flag  is  best  suited  to  given  conditions  and  the 
color  indicated  by  it  should  invariably  be  used.  For 
night  signaling,  the  acetylene  lantern  is  usually  em- 
ployed. Long-range  night  signaling  should  be  done 
with  the  searchlight  if  available.  The  employment 
of  the  semaphore,  in  daytime,  and  the  Ardois  system, 
at  night,  will  be  confined  to  more  or  less  permanent 
stations.  Rockets,  shells,  night  fires,  etc.,  are  only 
employed  for  special  or  emergency  signals. 

Miscellaneous  considerations. — For  various  reasons 
stations  should  not  be  located  at  or  near  camp  grounds. 
These  localities  usually  afford  mixed  backgrounds,  and 
the  presence  of  dust  and  smoke  and  the  interference 
caused  by  moving  bodies  of  troops  and  trains  will 


74 

militate  against  the  efficient  transmission  of  signals. 
Stations  located  in  vicinities  of  this  kind  are  also  sub- 
ject to  annoyance  from  noise  and  visits  of  unauthorized 
persons.  Signal  stations  should  be  convenient  for 
messenger  service  and  hence  as  near  commonly  trav- 
eled roads  as  the  physical  contour  of  the  country  will 
permit.  Locations  for  signal  stations  should  be  so 
selected  that  the  visual  lines  do  not  cross  traveled 
roads,  camps,  etc.,  as  dust  and  smoke  in  the  daytime 
and  lights  at  night  are  factors  in  determining  the  visi- 
bility of  signals.  Signal  stations  can  if  necessary  be 
artificially  concealed  by  erecting  screens  constructed 
of  limbs  of  trees,  etc.,  about  the  flanks  and  rear. 
Sheltered  positions  should  be  utilized  in  windy  weather. 
Intervisibility  table. — The  following  table  shows  the 
extent  of  horizon  for  different  heights  above  the  sea 
level — that  is,  it  shows  how  far  one  can  see  an  object 
which  is  itself  at  the  level  of  the  sea: 


Height  of  the  eye  above  sea 
level. 

Distance 

in  statute 

miles. 

Height  of  the  eye  above  sea 
level. 

Distance 

in  statute 

miles. 

10 feet                .                .  .. 

4 
5 

6 

8 

1? 

11 
12 
13 

115  feet 

14 

15  feet 

130  feet 

15 

20  feet 

150  feet 

16 

30  feet          

200  feet 

18 

40  feet 

230  feet 

20 

50  feet 

300  feet 

23 

60  feet 

350  feet 

25 

70  feet 

500  feet 

30 

85  feet.               .          .  .        . 

700  feet 

35 

100  feet 

900  feet 

40 

A  formula  to  determine  approximately  the  limits  of 
visibility  from  a  given  height  is  as  follows :  The  square 
root  of  the  height  of  the  station  in  feet  multiplied  by 
1.26  equals  the  distance  in  miles  at  which  the  signal  is 
visible. 


75 

Hence,  an  observer  whose  eye  is  30  feet  above  the 
sea  can  distinguish  an  object  7  miles  distant,  provided 
it  is  at  the  sea  level;  but  if  the  object  is  itself  15  feet 
above  the  sea  he  can  make  it  out  7  +  5=12  miles  off. 

FINDING   A    STATION. 

To  find  a  signalman  near  any  known  station,  note 
with  the  unaided  eye  some  prominent  landmark  near 
which  the  looked-for  person  or  object  is  supposed  to 
be,  and  direct  the  telescope  upon  the  place,  as  sight  is 
taken  over  a  gun  barrel,  covering  the  object;  if  the 
eye  is  now  placed  at  the  eyeglass  of  the  telescope,  the 
prominent  or  directing  landmark  will  be  found  in  the 
field  of  view.  It  will  be  easy  then  to  scale  the  country 
near  the  marker  until  the  signalman  is  found.  This 
method  is  often  necessary  at  night,  when  only  a  point 
of  light  is  seen  far  off  through  the  darkness,  and  the 
telescope  must  be  turned  upon  it.  When  the  compass 
bearing  of  the  object  sought  for  is  known,  the  telescope 
may  be  aligned  by  a  line  drawn  with  the  proper  com- 
pass bearing.  Commencing  then  with  the  view  at  the 
horizon,  the  telescope  is  slowly  moved  from  side  to 
side,  taking  in  fresh  fields  of  view  each  time  a  little 
nearer  to  the  observer,  until  the  whole  country  shall 
have  been  observed  from  the  horizon  to  quite  near  the 
station.  When  the  general  direction  only  of  the  object 
can  be  given  and  it  is  sought  for,  the  whole  landscape 
in  that  direction  to  the  horizon  should  be  divided  into 
sections  by  imaginary  lines,  the  limits  of  these  sections 
being  bounded  between  visible  landmarks  through 
which  the  bounding  lines  are  supposed  to  pass.  Each 
section  should  be  scrutinized  little  by  little  until  the 
glass  has  been  passed  over  every  spot.  Such  search 
will  seldom  fail  to  be  successful. 


76 

The  magnetic  bearings  of  all  stations  with  which 
another  station  has  worked  should  be  carefully  noted 
and  made  matter  of  record  in  the  office  directly  con- 
cerned, so  that  advantageous  use  may  be  made  of  this 
data.  In  addition,  guide  lines  may  be  established  by 
driving  two  stakes  firmly  into  the  ground  and  close  to 
each  other.  A  prolongation  of  a  line  through  the 
center  of  one  post  and  marked  on  the  adjacent  one  will 
strike  the  distant  station.  Under  each  line  should  be 
written  the  name  of  the  station  which  it  marks. 

Signalers  upon  permanent  or  semipermanent  sta- 
tions will  examine,  from  time  to  time,  every  promi- 
nent point  within  signal  distance,  to  see  if  communi- 
cation is  attempted  therefrom. 

Attempts  to  attract  the  attention  of  a  known  sta- 
tion, in  order  to  be  successful,  must  be  persistent. 
They  should  never  be  abandoned  until  every  device 
has  been  exhausted,  and  they  should  be  renewed  and 
continued  at  different  hours  of  the  day  and  night.  It 
must  be  remembered  that  efforts  which  have  failed 
because  the  observer's  attention  has  been  drawn  in 
another  direction  may  at  any  other  moment  be  suc- 
cessful if  the  observing  glass  chances  to  bear  on  the 
calling  signals. 

During  the  whole  time  that  signals  are  being  made 
to  attract  attention  the  calling  station  must  watch 
closely  with  the  telescope  the  station  called.  The 
watch  should  not  be  relaxed  until  communication  is 
established  or  the  station  ordered  abandoned. 

OPERATION    OF    STATIONS. 

Personnel. — At  signal  stations  where  continued 
operation  is  required  at  least  a  squad  or  ^^set  of  fours'' 
is  required.     Physical  and  mental  exhaustion  always 


77 


78 

result  from  continuous  signal  duty,  and  as  alertness 
of  mind  and  body  is  an  indispensable  factor  in  the 
prevention  of  errors,  two  reliefs  of  signalmen  should 
be  furnished  each  station  whenever  practicable. 
The  senior  olhcer  or  enlisted  man  is  in  charge  of  the 
station  and  is  responsible  for  efficiency  and  discipline. 
He  will  require  from  each  man  a  strict  and  entire 
attention  to  his  own  immediate  duties,  and  permit 
no  conversation  that  will  distract  the  men  at  work. 
He  will  be  careful  net  to  allow  persons  to  loiter  about 
the  station  or  within  the  hearing  of  tlie  words  called 
out  to  the  signaler.  The  assignment  of  men  should 
be  such  that  a  continuous  watch  for  signals  is  kept 
and  the  responsibility  for  neglect  to  promptly  answer 
calls  determined.  Of  the  station  men,  one  is  the 
sender,  whose  duty  it  is  to  transmit  all  signals  to  con- 
tiguous stations.  Another,  the  receiver,  attends  the 
telescope  and  reads  and  calls  off  the  signals  displayed 
at  the  distant  station.  A  third  man  acts  as  recorder, 
alternately  calling  oflF  the  outgoing  message  to  the 
sender  or  transcribing  the  incoming  message  repeated 
by  the  receiver. 

Calls  and  ^personal  signals, — Each  station  will  be 
assigned  a  call  consisting  of  one  or  two  letters.  Each 
and  every  operator  will  also  have  a  personal  signal 
of  like  character.  Station  calls  or  personal  signals 
when  once  given  or  assumed  will  not  be  changed 
except  by  order  of  higher  authority.  Ever}^  station 
should  at  all  times  have  on  hand  a  list  of  all  calls  and 
personal  signals  liable  to  be  encountered  in  station 
working.  The  general  call  suited  to  attract  the  atten- 
tion of  any  station  whose  regular  call  is  unknown 


79 

will  always  be  a  signal  represented  by  the  letter  ^^A^' 
in  the  Morse  or  the  letter  ^^E^^  of  the  Army  and  Navy 
Code. 

Opening  communication, — To  open  communication 
with  any  distant  station  whose  call  is  known,  signal 
the  call  repeatedly,  occasionally  signing  the  call  of 
the  home,  station.  If  the  regular  call  of  the  station 
sought  is  unknown  the  general  call  above  prescribed 
should  be  used.  As  soon  as  the  call  is  observed  the 
called  station  will  acknowledge  receipt  by  ''ii  ii/'  or 
'^I  understand/'  signing  thereafter  its  station  call. 
These,  preliminaries  completed,  the  stations  are  ready 
for  working. 

It  is  sometimes  difficult  to  secure  the  attention 
of  stations  at  unexpected  hours.  The  force  may 
not  be  strong  enough  for  an  uninterrupted  watch. 
To  provide,  so  far  as  possible,  for  this  contingency, 
it  may  be  concerted  that  if  communication  is  required 
at  unusual  time,  or  is  of  pressing  importance,  certain 
flags  shall  be  displayed,  rockets  discharged,  smokes 
shown,  or  other  attention-compelling  signals  used. 

When  a  number  of  stations  are  in  view  from  one 
station  and  it  is  desired  to  send  a  message  to  all  or 
more  than  one  station,  some  preconcerted  signal,  as 
a  rocket,  a  red  light,  or  some  peculiar  flag  or  torch 
signal,  should  be  designated  as  a  signal  for  general 
attention.  Upon  noticing  this  signal  all  the  called 
stations  reply,  and  then  observe  the  calling  station. 
This  plan  is  useful  when  two  or  more  stations  can,  at 
the  same  time,  read  the  signals  from  the  one  station, 
and  thus  together  receive  any  information  to  be 
transmitted  from  it. 


80 

When  a  signal  station  is  to  communicate  with  two 
or  more  stations,  a  telescope  should  be  firmly  fixed 
bearing  on  each,  when  practicable,  and  so  far  apart 
that  those  communicating  with  one  station  will  not 
disturb  the  other  party. 

Commencing  the  message, — Every  message  is  inva- 
riably commenced  by  the  signal  ^'Hr^^  or  ^^Anr.^^ 
Sometimes  at  the  commencement  of  communication 
a  preface  will  be  sent  in  order  to  give  some  preparatory 
information  to  the  receiving  station  regarding  the 
number  or  character  of  messages  about  to  be  sent. 
For  example,  '^Hr  8,''  means  ^^I  have  eight  for^you^' 
or  '^Hr  ck  300^'  means  a  three  hundred  word  message 
follows. 

Sending  and  receiving. — Before  the  commencement 
of  a  message,  care  should  be  taken  that  all  the  letters 
and  characters  thereof  are  entirely  and  correctly 
understood  by  the  signalman  whose  duty  it  is  to  call 
the  same  to  the  sending  operator.  The  message  is 
read  off  by  the  ' 'reader,''  who  first  calls  off  a  word 
and  then  spells  it  out  letter  by  letter.  The  ' 'reader'' 
should  observe  the  signals  of  the  operator  and  invite 
his  attention  to  any  apparent  errors.  When  the  last 
letter  of  a  word  is  announced  this  fact  will  be  com- 
municated to  the  sending  operator. 

At  the  receiving  station  the  man  at  the  telescope 
will  call  off  each  letter  as  received  and  not  wait  until 
the  completion  of  a  word.  On  reaching  the  end  of 
a  word  announcement  of  this  fact  will  be  made  to  the 
recorder. 

Breaking. — If  the  sending  operator  discovers  that 
he  has  made  an  error  which  will  probably  render  the 


81 

sense  of  the  message  unintelligible  at  the  receiving 
station,  he  will  make  the  signal  ^*BK^^  and  recom- 
mence the  message,  beginning  at  the  last  word  cor- 
rectly sent.  When  the  receiving  station  fails  for 
any  reason  to  get  correctly  what  is  being  sent,  the 
sending  station  is  interrupted  by  the  signal  ^^GA/' 
followed  by  the  last  word  correctly  received.  The 
message  will  then  be  recommenced  by  the  sending 
station  at  the  point  indicated. 

Discontinuance  of  transmission. — ^When  all  the 
messages  on  file  at  any  station  have  been  sent  the 
signal  '^NM'^  in  Morse  or  ^ Tease  signaling'^  in  the 
army  and  navy  system,  according  to  which  code  is 
authorized,  will  be  the  concluding  signal  of  the  send- 
ing station.  When  a  signal  station  is  operated  only 
during  the  daytime,  the  signal  'TN^'  will  be  trans- 
mitted after  all  business  filed  up  to  the  hour  desig- 
nated for  closing  has  been  dispatched. 

Aclcnowledgment  of  receipt. — No  message  will  be 
considered  sent  until  receipt  for  the  same  has  been 
acknowledged.  This  is  effected  by  making  either  the 
^^I  understands^  of  the  army  and  navy  or  the  'TK'^ 
of  one  of  the  Morse  systems,  depending  upon  the  one 
authorized.  In  every  case  the  receiving  operator's 
signal  is  signed  after  acknowledgment.  When  a 
number  of  messages  are  continuously  sent,  one 
acknowledgment  for  all  will  suffice  and  will  be  so 
understood.  In  receiving  messages  nothing  should  be 
taken  for  granted  and  nothing  considered  as  seen 
until  it  has  been  positively  and  clearly  in  view. 

Station  records. — Records  kept  at  field  signal  sta- 
tions will  be  confined  to  original  files  of  messages  sent 
40422—10 6 


82 

and  carbon  copies  of  messages  received.  Ordinarily 
the  only  available  stationery  will  be  the  United  States 
Army  Field  Message  Book.  Station  records  will  be 
invariably  preserved  as  part  of  the  station  equipment 
until  orders  for  their  disposition  are  given  by  higher 
authority.  Whenever  a  station  is  in  imminent  danger 
of  capture,  all  records  should  be  destroyed  in  the  dis- 
cretion and  under  the  direction  of  the  operator  in 
charge. 

Formation  of  signals. — Make  signals  with  regularity; 
do  not  send  one  word  rapidly,  the  next  slowly;  adopt 
such  a  rate  of  speed  as  can  be  read  by  the  distant 
signaler  without  causing  him  to  '^  break ^^  frequently. 
Make  a  distinct  pause  between  letters.  It  is  time 
gained  to  do  so ;  it  is  a  loss  of  time  and  an  annoyance 
to  run  letters  together.  Nothing  so  distinguishes  the 
good  from  the  indifferent  operator,  visual  or  telegraph, 
as  this.  When  signals  are  being  made  with  a  flag,  a 
fraction  of  a  second  will  be  ample.  In  using  the  lan- 
tern or  heliograph,  the  pause  between  letters  should 
be  relative  to  the  time  of  display  of  the  elements, 
longer  than  with  the  flag.  To  prevent  any  entangling 
of  the  flag  upon  its  staff,  skillful  handling,  acquired 
by  practice,  is  necessary.  It  is  accomplished  by  mak- 
ing a  scoop  of  the  flag  against  the  wind,  the  movement 
describing  an  elongated  figure  8,  thus  00-  The  mo- 
tions should  be  made  so  as  to  display  in  the  lateral 
waves  the  whole  surface  of  the  flag  toward  the  point 
of  observation. 

In  using  the  heliograph,  if  the  receiver  sees  that  the 
sender's  mirror  needs  adjustment,  he  will  turn  on  a 


83 

steady  flash  until  answered  by  a  steady  flash.  When 
the  adjustment  is  satisfactory,  the  receiver  wifl  cut 
off  his  flash  and  the  sender  wifl  resume  his  message. 

Repeating  the  message. — It  may  happen  that  very 
important  messages  received  by  signals  must  be  veri- 
fied by  repeating  back  from  the  receiving  station, 
signal  by  signal,  each  signal  used  by  the  sending 
station  in  conveying  the  message.  There  can  be  no 
error  in  signals  thus  verified,  and  the  correct  trans- 
mission of  the  message  is  made  certain.  For  such 
verification  each  signal  must  be  repeated  by  the  receiv- 
ing station  as  soon  as  it  is  made  at  the  sending  station. 

Signal  practice. — Full  efficiency  of  the  signaler  can 
be  maintained  only  through  constant  practice,  and 
those  in  charge  of  Signal  Corps  troops  should  see  that 
sufficient  practice  be  had  to  insure  that  accuracy  and 
rapidity  in  handling  messages  which  is  so  essential  in 
time  of  war. 

Instruction  should  commence  with  the  study  of  the 
principles  of  signaling  and  the  theories  of  their  general 
use,  and  the  pupil  should  be  well  grounded  in  this 
study  before  practice  is  begun.  He  should  so  memo- 
rize the  alphabets  to  be  used  that  no  letter  combina- 
tion will  require  thought  to  determine  its  meaning. 

Daily  inspections  should  be  made  to  insure  that  all 
signaling  instruments,  appliances,  and  materials  are  in 
readiness  for  instant  use.  Defects  in  the  apparatus 
annoy  the  sender;  to  a  greater  extent  they  annoy  the 
person  to  whom  the  messages  are  imperfectly  sent, 
and  delays  result  that  may  have  serious  consequences. 


84 
^      Chapter  VI. 

CODES  AND  CIPHERS. 

A  code  is  a  list  or  collection  of  arbitrary  words  or 
groups  of  letters  to  each  of  which  some  ordinary  word, 
proper  name,  phrase,  or  sentence  is  assigned  for 
meaning. 

Ciphers  embrace  all  means  whereby  writings  may 
be  transcribed  into  occult  terms.  All  ciphers  employ 
some  distinct  method  for  transcription,  which  method 
is  termed  a  key.  In  practice  the  key  is  usually 
applied  directly  in  enciphering  and  reversed  in  deci- 
phering messages. 

CODES   IN   USE. 

The  codes  of  the  Western  Union  and  Postal  Tele- 
graph companies  are  examples  of  well-known  codes 
suited  to  general  commercial  use.  Besides  these, 
many  special  codes  have  been  formulated,  so  as  to 
embody  technical  expressions  especially  adapted  to 
use  in  particular  lines  of  industry.  The  War  Depart- 
ment Code  is  a  military  code  adapted  to  the  special 
needs  of  the  military  establishment  in  peace  and  war. 

EMPLOYMENT   OF   CODES. 

Codes  are  primarily  intended  for  economy,  but  they 
may  also  be  readily  employed  to  secure  secrecy.  When 
used  solely  for  economy,  the  coded  message  is  said  to 
be  plain  code ;  that  is,  the  word  or  phrases  of  the  mes- 
sage are  coded  by  direct  reference  to  their  respective 
code  equivalents.  Thus  plain  code  is  readily  trans- 
latable to    anyone    in    possession    of    a  code  book. 


85 

When  secrecy  is  desired,  some  methctd  of  enciphering 
or  key  is  employed  in  such  a  way  that  only  persons 
in  possession  of  it  can  in  conjunction  with  the  code 
book  decipher  it.  In  such  case  the  message  is  said 
to  be  in  cipher  code. 

CIPHER   CODE. 

In  all  codes  each  expression  and  its  equivalent  in 
plain  language  is  assigned  a  number.  These  num- 
bers usually  commence  at  unity  and  increase  consecu- 
tively to  any  desired  figure.  Messages  may  be 
enciphered  by  means  of  a  key  number  or  series  of  num- 
bers. An  additive  number,  say  55  additive,  requires 
that  in  enciphering  a  message,  the  fifty-fifth  word 
numerically  greater  than  the  proper  code  word  shall 
be  used;  if  55  subtractive  is  used,  the  fifty-fifth  word 
numerically  smaller  than  the  proper  code  word  is  to 
be  used.  By  agreement  a  single  key  number  can  be 
used  alternately  additive  and  subtractive,  that  is,  first 
additive,  second  subtractive,  third  additive,  etc. 

The  key  numbers  are  used  over  and  over  until  the  en- 
tire message  is  enciphered.  The  key  number  can  some- 
times be  expressed  by  a  single  word,  as,  for  instance, 
^^Grant,^^  each  letter  having  a  value  of  tens  in  accord- 
ance with  its  position  in  the  alphabet;  that  is,  G,  the 
seventh  letter  equals  70;  II  equals  180;  A  equals  10; 
N  equals  140;  and  T  equals  200.  Or  by  preconcerted 
arrangement  letters  may  represent  units  or  hundreds. 
Security  from  translation  by  persons  not  having  the 
key  number  is  greater  when  the  key  numbers  are  used 
alternately  additive  and  subtractive.  If  a  cipher 
key  word  is  used,  it  should  be  one  of  an  odd  number  of 


86 

letters,  as,  for  instance,  '^Jones,^^  the  numbers  corre- 
sponding to  the  positions  of  the  letters  in  the  alphabet. 
The  first  number  should  be  additive,  the  second  sub- 
tractive,  etc.  By  this  means  the  first  letter  of  the  key 
word  is  additive  the  first  time  it  is  used,  subtractive 
the  second,  additive  the  third,  and  so  on.  In  some 
instances  the  key  number,  when  added  to  or  sub- 
tracted from  the  code  number,  gives  a  resulting  num- 
ber exceeding  the  highest  code  number  or  less  than 
unity.  In  cases  of  this  kind  it  should  be  remembered 
in  enciphering  that  unity  follows  the  highest  code 
number  in  addition,  and  that  the  highest  code  num- ' 
ber  follows  unity  in  subtraction.  In  deciphering  a 
message  the  process  of  enciphering  is  reversed. 

THE    WAR   DEPARTMENT   CODE. 

As  previously  stated,  the  War  Department  Code 
is  the  technical  military  code  and  contains  expres- 
sions numbered  consecutively  from  1  to  62,000. 
All  the  code  words  are  composed  of  6  letters,  which  are 
so  arranged  that  the  vowels  and  consonants  invariably 
alternate.  In  the  formation  of  code  words  the  fol- 
lowing 13  letters  only  are  used,  viz.  A,  B,  D,  E,  F,  G, 
I,  K,  M,  N,  S,  U,  and  X.  The  body  of  the  code  book 
is  arranged  as  follows: 

(a)  Army  list,  containing  the  name  of  every  commissioned 

officer  in  the  regular  establishment. 
(6)  Military  organizations,  giving   all   batteries,  companies, 

troops,  etc. 

(c)  Military  posts  and  stations,   covering  Alaska,   Hawaii, 

Philippine  Islands,  Porto  Rico,  and  the  United  States. 

(d)  United  States  naval  stations  and  vessels. 

(e)  Geographical  names. 


87 

(/)   Miscellaneous  tables  as  tollows: 

Numerals. 

Arrivals  and  departures. 

Dates. 

Indorsements. 

Letter  acknowledgments. 

Requisitions. 

Telegram  acknowledgments. 

Mails,  shipments,  and  transports. 

Blanks  for  future  additions  as  they  may  be  needed. 

Ranks  and  grades  of  officers  and  men  in  the  Army . 

Wireless  stations  of  the  Army  and  Navy. 
{g)  Alphabetical  list  of  code  expressions  arranged  conven- 
iently for  use. 

When  it  is  desired  to  transmit  some  word  or  expres- 
sion not  to  be  found  in  the  code  and  no  suitable  syn- 
onym can  be  discovered  the  word  or  expression  should 
be  sent  in  plain  language  or  spelled  out  by  the  equiv- 
alents for  letters  and  endings  to  be  found  on  page  589. 

Complete  instructions  for  the  use  of  the  code  either 
as  a  code  or  cipher  are  contained  in  the  introductory 
pages  of  the  book. 

CIPHER   CODE    IN   FIELD   WORK. 

The  use  of  cipher  code  in  enciphering  field  messages 
will  usually  be  practicable  only  between  the  several 
headquarters  and  other  large  stations  supplied  with 
code  books.  This  method,  too,  is  prohibitive  for  urgent 
messages  when  the  time  of  enciphering  and  decipher- 
ing is  an  important  factor  connected  with  delivery. 

FIELD    CIPHERS. 

Description  and  use, — Field  ciphers  include  all  sys- 
tems and  the  apparatus  connected    therewith  which 


88 

are  ordinarily  employed  in  enciphering  and  decipher- 
ing field  messages.  Field  ciphers  are  intended  for  use 
when  code  books  are  not  available,  and  hence  the  em- 
ployment of  cipher  code  is  precluded.  Some  methods 
of  field  cipher  employ  simple  forms  of  apparatus,  while 
others  require  the  use  of  no  apparatus  at  all. 

Forms  of  field  cipher. — There  are  two  general  classes 
of  field  cipher.  The  first  class  employs  the  transpo- 
sition or  reversal  of  the  letters  or  words  of  a  message 
according  to  some  preconcerted  rule  as  a  means  of 
secrecy.  The  route  cipher  hereafter  described  is  an 
example  of  this  class.  The  method  used  in  ciphers  of 
the  second  class  consists  in  the  substitution  of  certain 
letters  or  symbols  for  each  of  the  individual  letters 
composing  the  words  of  the  message.  Both  classes  of 
cipher  can  be  rendered  more  efficient  by  a  judicious 
use  of  inversions  and  by  the  concealment  of  termina- 
tions. 

Inversions. — By  the  inversions  of  the  whole  or  cer- 
tain parts  of  messages,  according  to  some  preconcerted 
arrangement,  the  complications  of  cipher  can  be  greatly 
increased.  If  a  message  is  to  be  inverted,  either  as  a 
whole  or  by  clauses,  it  should  be  inverted  before  the 
cipher  letters  are  written  over  it.  Messages  may  be 
further  complicated  by  sending  the  letters  of  each 
word  backward  in  various  other  prearranged  combi- 
nations. 

Concealment  of  terminations. — To  evade  the  discov- 
ery of  the  key  or  keys  employed,  it  is  most  important 
that  the  termination  of  the  words  of  a  message  should 
be  concealed.  The  best  method  to  conceal  the  begin- 
ning, and  at  the  same  time  the  termination  of  words. 


89 

is  to  divide  them  into  arbitrary  groups  of  four  or  five 
letters  edch.  This  procedure  will  add  immeasurably 
to  the  strength  of  the  cipher  and  should  in  no  way 
confuse  one  in  possession  of  the  key.  For  instance, 
the  words  ^'sufficient  time''  would  be  divided  "suff 
'4cie''  ^'ntti''  ''me/'  and  such  blind  letters  as  may  be 
agreed  upon  to  fill  the  last  two  spaces  of  the  last  group. 
All  such  artifices  as  this  will  surely  delay  a  translator 
not  in  possession  of  the  key. 

CIPHER    APPARATUS. 

TTie  cipher  disk, — The  cipher  disk  is  composed  of 
two  disks  of  cardboard,  leather,  or  other  material 
joined  concentrically,  the  upper  disk  revolving  upon 
the  lower.  The  alphabet,  reading  from  left  to  right, 
and  such  other  signals,  numerals,  or  combinations  of 
letters,  as  may  be  desired,  are  printed  around  the 
circumference  of  the  lower  disk.  On  the  upper  disk 
are  printed  the  alphabet  and  such  other  signals,  nu- 
merals, or  combinations  of  letters  as  are  printed  on 
the  lower  disk.  On  the  lower  disk  they  are  printed 
from  left  to  right,  while  on  the  upper  disk  they  are 
printed  from  right  to  left.  If  it  is  desired  to  encipher 
a  message,  the  key  letter  or  "the  first  letter  of  the  key 
word  or  words  is  set  opposite  "A."  Let  us  assume 
it  to  be  "J."  The  cipher  letters  to  be  written  are 
those  opposite  the  text  letter  when  the  letter  "a"  on 
the  upper  disk  is  set  opposite  "J"  on  the  lower  disk. 
For  example,  "Send  powder"  would  be  written  "rfwg 
uvngfs." 

Having  a  cipher  disk  as  above  described,  this  mere 
transposition  of  letters  would  delay  but  a  short  time 


90 

the  deciphering  of  a  message  by  one  not  knowing  the 
key  letter,  as  it  would  be  necessary  only  to  place,  in 
turn,  opposite  ^^a'^  each  of  the  letters  of  the  alphabet 
beginning  with  ^^b^^  and  noting  the  letters  opposite 
the  enciphered  letters.  But  this  simple  disk  can  be 
used  with  a  cipher  word,  or  preferably,  cipher  words 
known  only  to  the  correspondents,  and  it  is  entirely 
improbable  that  a  message  so  enciphered  could  be 
deciphered  in  time  to  be  of  any  value  to  the  enemy. 
Using  the  key  words  ^^  permanent  body^^  to  encipher 
the  message  '^Reenforcements  will  reach  you  at  day- 
light,'^ we  would  proceed  as  follows:  Write  out  the 
message  to  be  enciphered  and  above  it  write  the  key 
word  or  key  words,  letter  over  letter,  thus : 

PERMANENTBODYPERMANENTBODYPERMANENTB 
Reenforcementswi  1  Ir  eachyouatdayl  i  ght 
yanzvznlppkqfxijbBpwanruqpeplomccwhmi 

Now  bring  the  ^^  a''  of  the  upper  disk  under  the  first 
letter  of  the  key  word  on  the  lower  disk,  in  this  case 
^^  P.''  The  first  letter  of  the  message  to  be  enciphered 
is  ^^  R. ''  ^'  y  is  found  to  be  the  letter  connected  with 
'^  R  '^  and  it  is  put  down  as  the  first  cipher  letter.  The 
letter  ''a"  is  then  brought  under  ''E,''  which  is  the 
second  letter  of  the  key  word.  ^^E'^  is  to  be  enci- 
phered and  ^^  a^'  is  found  to  be  the  second  cipher  letter. 
Then  bring  ''a''  to  ''R''  and  the  cipher  letter  will  rep- 
resent ^^e,''  the  third  text  letter  of  the  message.  Pro- 
ceed in  this  manner  until  the  last  letter  of  the  cipher 
words  is  used,  and,  beginning  again  with  the  letter 
^^P,''  so  continue  until  all  letters  of  the  message  have 
been  enciphered.  Divided  into  groups  of  four  letters, 
it  will  be  as  follows :  ''yanz  vznl  ppkq  fxij  bpwa 
nruq     pepl     omcc     whmi.^' 


91 


Fig.  19.— Cipher  disk. 


92 

To  decipher  the  message,  reverse  the  proceedings 
above  described;  thus  the  letter  ^^a^^  on  the  upper 
disk  is  brought  under  the  first  letter  of  the  key  word 
^^P/'  Following  these  instructions,  we  find  the  first 
cipher  letter  of  the  message;  '^a''  is  then  brought  to 
the  next  letter  of  the  key  word.  In  this  case  ^^E'^  is, 
of  course,  the  next  letter  of  the  text.  ^'R^'  is  the 
next  letter  in  the  key  and  ''a^'  is  brought  over  it. 
The  cipher  letter  ^^n''  gives  us  the  next  text  letter, 
which  is  ^'e,  ^'  and  so  on  until  the  completion  of  the 
message.  If  the  letters  of  the  key  word  or  phrase  are 
exhausted,  begin  again  with  the  first  letter  and  so 
continue  until  the  entire  message  is  deciphered. 

With  a  key  word,  or,  preferably,  a  key  phrase  of 
three  or  four  words,  the  deciphering  of  a  message  is 
extremely  difficult. 

In  a  military  cipher  message,  it  may  be  desired  to 
transmit  numerals,  the  spelling  out  of  which  would 
require  considerable  time.  This  can  be  done  by  an 
arrangement  of  the  cipher  disk  so  that  the  numerals 
of  which  will  appear  in  the  same  order  as  and  follow 
the  letters  of  the  alphabet.  Thus  on  the  lower  disk  1 
is  placed  opposite  A;  2  opposite  B;  3  opposite  C;  4 
opposite  D;  5,  6,  7,  8,  9,  and  0  opposite  E,  F,  G,  H, 
I,  and  J,  respectively. 

On  the  upper  disk  the  above  numerals  also  appear, 
beginning  numeral  1  opposite  A;  2  opposite  B,  etc., 
0  being  opposite  J. 

The  arbitrary  sign  XX  will  be  used  to  indicate 
^^ numerals  follow^'  and  ^^ numerals  end.^'  Supposing 
then  we  wish  to  send  the  following  message:  ^^  Send 
6,000  cavalry  at  once,^'  and  that  the  key  word  was 


93 

''Washington/^  Following  the  instructions  hereto- 
fore given  for  enciphering,  we  would  place  the  words 
as  follows: 

WASHINGTONWAS  H I N  G  T  ONWASHI 
SENDXX  6000  X  X  CAVAL  R  YAT  ONCE 
EWFELQBKFEZDQ  H  NNYCQNDMFFE 

In  place  of  a  disk  means  may  be  extemporized  by 
taking  two  strips  of  paper,  on  one  of  which  the  alpha- 
bet, numerals,  etc.,  are  twice  written  in  succession. 
On  the  other,  with  equal  spacing,  the  alphabet,  etc., 
are  written  once,  but  in  reverse  order.  By  sliding 
these  strips  in  juxtaposition  with  each  other  they  will 
replace  the  disk. 

Cipher  disks  should  never  be  allowed  to  fall  into  the 
hands  of  the  enemy  or  of  anyone  unauthorized  to  have 
and  use  them;  to  insure  this,  special  instructions 
should  be  issued  for  their  care  and  keeping. 

THE   MATHEMATICAL    CIPHER. 

This  cipher  is  a  highly  efficient  one  for  the  purpose 
of  secrecy  and  at  the  same  time  requires  no  apparatus 
whatever  attendant  upon  its  use.  The  cipher  is  con- 
structed as  follows:  Commit  to  memory  the  alphabet 
by  numbers,  viz.  A,  1 ;  B,  2 ;  etc.  Take  any  key  word, 
phrase,  or  sentence  desired;  for  example,  '^A  dis- 
covery.''  Suppose  the  message  to  be  enciphered  is 
''Send  me  powder  tonight.^'  The  enciphering  of  the 
message  using  the  key  given  above  will  be  as  follows : 

To  encipher,  first  write  out  the  key,  letter  by  letter, 
placing  the  message  letter  by  letter  beneath  it.  Then 
reduce  the  letters  of  the  key  and  the  message  to  the 


94 

numeral  alphabetical  equivalents.  Add  the  individual 
columns  and  subtract  unity  from  each.  From  any 
result  thus  found,  which  exceeds  the  number  of  letters 
in  the  alphabet,  the  number  26  must  be  subtracted. 
The  final  totals  reduced  to  letters  by  numerical  alpha- 
betical equivalents  will  then  give  the  cipher. 

AD  IS  COVERYADISCOVER 
sendmepowdertonight 

which  reduced  to  numerical  equivalents  according  to 
alphabetical  position  of  letters  becomes: 

14   9  19   3  15  22   5  18  25  1   4   9  19   3  15  22  5  18 
19  5  14   4  13   5  16  15  23   4  5  18  20  15  14   9   7  8  20 

Now  add  the  columns  and  subtract  unity  from 
each.  If  any  result  so  found  exceeds  the  number  of 
letters  in  the  alphabet  26  must  be  subtracted  from  it. 

In  the  example  given  the  numerical  totals  are  as 
follows : 


20  9 
1  1 

23 

1 

23  16  20 
1   1   1 

38 

1 

20 

1 

41 

1 

29 

1 

6  22 
1   1 

29 

1 

34 
1 

17  24 
1   1 

29 
1 

13 

1 

38 

1 

19  8 

22 

22  15  19 

37 

26 

19 

40 

26 

28 
26 

5  21 

28 
26 

33 

26 

16  23 

28 
26 

12 

37 
26 

19  8  22  22  15  19  11  19  14   2  5  21   2   7  16  23   2  12  11 

which  connected  to  letters  gives: 

SHVVOSKSNBEUBGOWBLK 

the  cipher  required. 

Translation  of  cipher  is  had  by  reversing  the  proc- 
esses described. 

THE    ROUTE    CIPHER. 

This  is  a  cipher  in  which  the  words  or  a  message 
are  retained  unchanged,  but  are  so  disarranged  by 


95 

preconcerted  rules  that  the  sense  becomes  uninteUigi- 
ble.  The  message  as  received  seems  to  be  a  number 
of  disconnected  words  and  without  meaning,  but  by 
arrangement  in  proper  order  in  accordance  with  cer- 
tain rules  can  be  easily  read.  Messages  enciphered 
in  this  manner  may  be  translated  by  persons  not  in 
possession  of  the  key,  and  therefore  the  information 
contained  therein  should  only  be  of  such  a  character 
as  to  be  of  little  value  to  the  enemy  unless  acted  upon 
immediately.  The  usual  method  employed  in  arrang- 
ing a  message  for  this  cipher  is  to  write  the  words  in 
vertical  columns.  The  number  of  words  in  each 
column  should  always  equal  the  number  of  columns, 
being  made  so,  if  necessary,  by  the  addition  of  sufficient 
'^ blind''  words.  A  preconcerted  route  is  agreed  upon, 
as  up  to  the  first  column,  down  the  third,  up  the 
second,  etc.  The  message  is  then  transmitted  without 
reference  to  the  columns,  but  is  deciphered  at  the 
receiving  station  by  column  arrangement  and  perusal 
along  the  original  route. 

For  example,  to  encipher  the  message  ^^Move  day- 
light. Enemy  approaching  from  north.  Prisoners 
say  strength  one  hundred  thousand.  Meet  him  as 
planned,"  arrange  as  follows: 

Move  strength  planned  say 

dayHght  one  as  prisoners 

enemy  hundred  him  north 

approaching  thousand  meet  from 

Here  the  route  is  down  the  first  column,  up  the 
fourth,  down  the  second,  and  up  the  third. 


.       96 

CIPHER   DETECTION. 

General  instructions, — In  deciphering  a  message  in 
which  the  same  cipher  letter  or  symbol  is  uniformly 
used  to  represent  the  same  text  letter,  the  following 
data  will  be  of  assistance. 

The  proportion  of  occurrence  of  letters  of  the  alpha- 
bet in  English  words  is  as  follows :  For  every  2  of  the 
letter  Q  there  are  4  of  the  letter  X,  8  of  K,  16  of  B, 
13  of  C,  80  of  I,  N,  O,  and  S;  85  of  A,  90  of  T,  and 
120  of  letter  E. 

The  compounds  most  frequently  met  with  are  NG 
EE  LL  MM  TT  DD  and  NN. 

The  order  of  frequency  in  which  the  letters  of  the 
alphabet  occur  as  initial  letters  in  words  is  as  follows : 

S,  C,  P,  A,  D,  I,  F,  B,  L,  T. 

Employment  of  Cipher  DisTc. 

If  messages  are  enciphered  by  a  mere  transposition 
of  the  letters  of  the  alphabet,  the  cipher  disk  can  be 
used  to  quickly  decipher  the  message,  as  the  following 
example  will  show:  Assuming  that  F  is  used  to 
represent  A,  G  to  represent  B,  H  to  represent  C, 
I  to  represent  D,  J  to  represent  E,  etc.,  in  regular 
sequence,  and  that  the  message  to  be  enciphered  is: 
'^We  are  short  of  rifle  ammunition;  send  30,000 
rounds  at  once.^' 

This  would  be  enciphered  if  divided  into  groups  of 
four  letters  as  follows : 

jbfo    bnyr    omra    oxub    fuls    xmxr    snbs    cmjb    smhm     yrhi 
fsco      rise       nfmr     sdb. 


97 

Place  "si^^  of  the  upper  cipher  disk  under  B  of  the 
lower  disk  and  notice  whether  the  cipher  letters 
jbfo — the  first  group — are  intelligible.  They  give 
^'sawn/'  continue  this  for  '^saw/'  the  first  three  letters, 
may  be  the  text  word.  Now  the  next  group  is 
B  N  Y  R  and  these  give  A  O  D  K.  We  know  that  A 
does  not  represent  B  because  the  first  8  cipher  letters 
give  the  meaningless  letters  ^^sawnaodk.'^  Turn  ^'a^' 
to  C  and  we  have  for  the  first  group  T  B  X  O,  which 
is  without  meaning.  Turning  ^^a^'  to  D  we  get 
U  C  Y  P,  a  meaningless  jumble.  Turn  '^a'^  to  E  and 
we  get  V  D  Z  Q,  which  is  meaningless.  Now  turn 
'^  a'^  under  F  and  we  find  that  JBFO  mean  ^^  Wear/^ 
which,  so  far  at  least,  gives  us  a  part  of  a  word,  or  the 
word  ^^We'^  arid  part  of  another  word.  We  con- 
tinue to  the  next  group  B  N  Y  R,  which  gives  us 
'^esho.^'  We  now  have  these  letters  ^^Wearesho,'^ 
which  at  a  glance  we  read  ^^We  are  sho;^^  continuing 
to  the  next  group  O  M  R  A  the  cipher  disk  gives  us 
'^rtof,^'  and  we  read  ^^We  are  short  of''  and  know  we 
have  found  the  key  letter,  and  the  information  hidden 
in  the  cipher  is  ours.  Continue  deciphering  with  ^^  a" 
under  F  until  the  end  of  the  message.  Sometimes 
the  key  letter  is  changed  after  two,  three,  or  four 
letters. 

It  is  a  matter  of  minutes  only  to  run  through  the 
alphabet  and  learn  the  meaning  of  a  message  so 
enciphered. 

40422—10 7 


98 
Chapter  VII. 

FIELD  GLASSES  AND  TELESCOPES. 

Reflection — refraction — lenses. 

When  light  falls  on  a  transparent  •  body,  part  is 
reflected  and  part  is  refracted.  The  angle  which  the 
ray  makes  with  the  normal,  or  perpendicular,  to  the 
surface  at  the  point  of  contact  is  known  as  the  angle 
of  incidence,  and  the  angles  which  the  reflected  and 
refracted  rays  make  with  the  same  normal  are  known 
respectively  as  the  angle  of  reflection  and  refraction. 
The  reflected  ray  makes  the  same  angle  with  the  nor- 
mal as  the  incident  ray,  while  the  refracted  ray,  when 

passing  from  a  rarer  to  a 
denser  medium,  is  bent 
toward  the  normal,  and 
vice  versa;  the  denser  the 
Fig.  20.  medium  into  which  the 

ray  passes  the  greater  is 
the  deviation.  This  law  allows  us  at  once  to  understand 
the  action  of  a  lens,  which  may  be  defined  as  a  trans- 
parent medium  that  from  the  curvature  of  its  surface 
causes  the  rays  of  light  traversing  it  to  either  con- 
verge or  diverge.  The  ordinary  lenses  have  either 
spherical  surfaces  or  a  combination  of  spherical  and 
plane  surfaces.  This  combination  will  give  rise  to  six 
classes  (fig.  20):  (a)  Double  convex;  (6)  piano  con- 
vex; (c)  double  concave;  (d)  piano  concave;  (e)  con- 
verging, and  (/)  diverging  meniscus.  Those  lenses 
which  are  thicker  at  the  center  than  at  the  edges  are 
converging  or  concentrating  lenses,  and  those  which 
are  thicker  at  the  edges  than  the  center  are  diverging. 


99 

FOCUS OPTICAL    CENTER. 

The  focus  of  a  lens  is  the  point  where  the  refracted 
rays  or  their  prolongation  meet;  if  the  rays  themselves 
intersect  after  refraction  the  focus  is  real,  and  if  their 
prolongations  meet  the  focus  is  virtual.  The  line 
passing  through  the  centers  of  curvature  of  the  two 
surfaces  of  a  lens  is  called  the  principal  axis  and  con- 
tains a  point  known  as  the  optical  center,  which  has  the 
property  by  virtue  of  which,  if  a  ray  passes  through 
it,  the  ray  will  not  be  deviated.  The  optical  center 
can  always  be  found  by  drawing  two  radii  parallel  to 
each  other,  one  from  each  center  of  the  curvature  of 
the  surface  until  the  radii  intersect  their  respective 
surfaces,  then  draw  a  line  joining  these  two  points. 
The  intersection  of  this  last  line  with  the  principal 
axis  will  give  the  optical  center. 

IMAGE CONJUGATE    FOCI. 

Let  AB  be  the  section  of  a  double  convex  lens  and 
C  and  D  (fig.  21)  be  the  centers  of  curvature  of  the 
two  surfaces.  Draw  the  lines  CD'  and  DE  from  C  and 
D  parallel  to  each  other,  then  join  D'  and  E  by  a 
straight  line.  The  point  O  will  be  the  optical  center 
of  the  lens.  Let  us  take  a  point  R,  on  the  principal 
axis  as  a  source  of  light;  the  ray  RD  passes  through 
the  optical  center  and  is  not  deviated.  The  ray  RK 
on  striking  will  be  refracted  in  the  direction  KG 
toward  the  perpendicular  to  the  surface  KD  in  accord- 
ance with  the  law  of  refraction,  as  glass  is  denser  than 
air.  On  emerging  at  G  it  is  refracted  away  from  the 
perpendicular  to  the  surface  CG,  since  it  passes  from  a 


100 

denser  to  a  rarer  medium,  and  will  intersect  the  ray 
RD  at  the  point  R'.  In  a  Hke  way  the  ray  RK'  will 
be  found  to  intersect  the  ray  RD  at  the  same  point, 
R',  which  is  the  focus  for  all  rays  coming  from  R.  The 
point  R^  is  said  to  be  the  image  of  the  object  R,  and 
when  the  two  points  are  considered  together  they  are 
called  conjugate  foci.     If  the  incident  beam  is  com- 


posed of  parallel  homogeneous  light,  the  rays  will  all 
be  brought  to  a  focus  at  a  point  on  the  principal  axis, 
called  the  principal  focus  of  the  lens,  and  the  distance 
of  this  point  from  the  optical  center  is  the  principal 
focal  length,  which  is  always  a  fixed  quantity  for  any 
given  lens. 

LAW    OF   FOCI. 

There  is  a  fixed  relation  between  the  principal  focal 
length  of  a  double  convex  lens  and  the  position  of  the 
image  of  the  object  which  may  be  expressed  as  fol- 
lows: -^  =  ^  — — ,  in  which  i  and  o  are  the  distances  of 
%     f      0 

the  image  and  object,  respectively,  from  the  optical 
center  and /the  focal  length,  from  which  we  see  that  for 
all  positions  of  the  object  from  an  infinite  distance 
away  from  the  lens  to  double  the  principal  focal  distance. 


101 


the  image  will  be  on  the  other  side,  between  a  distance 
equal  to  the  principal  focal  length  and  double  this 
length.  These  are  the  limits  of  the  image  and  object  in 
the  ordinary  cases.     If  we  place  this  expression  in  the 

following  form: -1  = -5^,  and  suppose   the    object   to 

remain  the  same  distance  from  various  lenses,  it  will 
be  seen  that  the  image  will  be  closer  to  the  lens  which 
has  the  shorter  focal  length.  The  principal  focal  dis- 
tance, or,  briefly,  the  focal  length  of  the  lens,  depends 
on  the  curvature  of  the  surfaces,  and  the  greater  the 
curvature  the  shorter  the  focal  length. 

FORMATION    OF   IMAGE. 

Let  us  now  see  how  an  image  is  formed  by  a  convex 
lens,  and  suppose  that  CD  is  the  section  of  a  double 
convex  lens  (fig.  22),  O  the  optical  center,  and  AB  an 


Fig.  22. 

object  at  a  greater  distance  from  the  optical  center 
than  double  the  focal  length.  Kays  will  pass  out  in 
all  directions  from  the  object  and  some  will  fall  on  the 
lens.  A  ray  from  A  will  pass  through  the  optical  cen- 
ter and  will  not  be  deviated;  others  will  be  incident 
at  various  points,  for  example,  E  and  G,  and  if  we 


102 

apply  the  law  of  refraction  we  will  find  that  AE  and 
AG  will  intersect  each  other  and  AO  at  the  point  A', 
provided  we  do  not  consider  the  figure  of  the  lens, 
forming  one  point  of  the  image  A'  B';  similarly  for 
rays  from  other  points  of  the  object,  as,  for  example,  B, 
we  can  construct  the  focus  B',  and  thus  obtain  the 
image  A'  B',  which  is  inverted  and  smaller  than  the 
object  AB.  The  relative  size  of  the  image  and  object 
will  be  directly  as  the  conjugate  foci,  and  these  can  be 
found  at  once  from  the  equation  of  the  lens. 

SPHERICAL    ABERRATION. 

If,  however,  we  consider  the  form  of  the  lens,  we 
will  find  that  all  the  rays  emerging  from  one  point  on 
the  object  are  not  brought  to  the  same  focus,  because 
the  rays  incident  on  the  edges  of  the  lens  are  refracted 
to  a  greater  extent  than  those  falling  on  the  center, 
and  will  be  brought  to  a  focus  at  a  shorter  distance  from 
the  lens  than  those  passing  through  the  central  part. 
This  confusion  or  wandering  of  the  foci  from  one  point 
is  called  spherical  aberration,  or  aberration  of  form,  and 
is  due  solely  to  the  geometrical  form  of  the  lens. 

CHROMATIC    ABERRATION. 

In  what  has  been  said  about  the  visual  image  we 
have  supposed  that  the  light  was  monochromatic,  or 
homogeneous.  Let  us  see  what  will  happen  if  the 
light  is  polychromatic,  say,  for  example,  sunlight,  and 
let  a  beam  of  sunlight  be  intercepted  on  a  screen  after 
passing  through  a  double  convex  lens.     It  will  be 


103 


observed,  as  in  figure  23,  that  the  violet  rays  are 
brought  to  a  focus  nearest  the  lens,  and  the  red  farthest 
away,  and  circles  of  light  will  be  seen  on  the  screen; 
this  wandering  of  the  colored  rays  from  a  common 


Fig.  23. 

focus  is  called  chromatic  aberration  and  depends  on 
the  dispersive  properties  of  the  material  of  which  the 
lens  is  made.  Here  is  a  defect  that  can  not  be  cor- 
rected by  a  stop,  but  as  the  refractive 
and  dispersive  properties  of  a  substance 
do  not  vary  together,  it  is  possible  to 
combine  two  substances,  one  with  high 
refractive  and  low  dispersive  properties 
and  the  other  with  the  reverse  proper- 
ties. If  proper  curves  are  given  to  them 
they  will  correct  each  other,  thereby 
producing  coincidence  of  the  visible  and 
chromatic  foci.  Such  a  combination 
gives  an  achromatic  lens,  which  is  usu- 
ally composed  of  a  double  convex  of 
crown  glass  cemented  to  a  diverging  meniscus  of  flint 
glass,  as  shown  in  section  in  figure  24.  This  combi- 
nation is  not  absolutely  achromatic,  but  sufficiently 
so  for  all  general  purposes. 


Fig.  24. 


104 

TELESCOPES. 

The  telescope  is  an  optical  instrument  based  on  an 
object  glass  or  reflector  to  form  a  real  image  of  a  real 
and  distant  object,  and  of  an  ocular  to  magnify  and 
view  the  image.  Telescopes  are  classified  as  refracting 
or  reflecting  according  as  the  object  glass  is  a  lens  or  a 
reflector.  The  object  glass  must  be  essentially  con- 
vex if  the  telescope  is  a  refractor,  and  if  a  reflector, 
the  object  mirror  must  be  concave;  the  ocular  may  be 
either  concave  or  convex. 

There  are  four  types  of  refractive  telescopes  used  for 
miHtary  purposes,  viz: 

1.  The  astronomical. 

2.  The  terrestrial. 

3.  The  galilean. 

4.  The  prismatic. 

Figure  26  is  a  section  of  an  astronomical  telescope. 
The  object  glass  {D)  is  a  combination  consisting  of  a 
double  convex  and  a  double  concave  lens  cemented 
together  with  Canada  balsam.  The  double  concave 
lens  is  added  to  correct  for  chromatic  aberration.  The 
ocular  {E)  is  a  convex-concave  lens. 

Rays  of  light  from  some  distant  object  are  converged 
by  the  objective  {D)  and  form  an  inverted  image  {ab) 
at  the /ocaZ  plane  (F).  The  eye  lens  {E)  receives  the 
divergent  pencils  from  a  and  h  and  bend  them  so  that 
they  enter  the  eye  as  if  coming  apparently  from,  the 
direction  oi  a'  V  where  the  apparent  image  is  seen. 
From  the  eyepiece  {E)  the  rays  emerge  in  a  cone  of 
pencils  of  light  smaller  than  the  pupil  of  the  eye,  which 
enables  a  telescope  of  this  type  to  have  a  large  field  of 
view.     The  image,  however,  is  inverted  and  the  astro- 


105 


nomical  telescope  in  its  original  form  is  therefore  not 
suitable  for  military  purposes.  In  a  modified  form 
it  is  much  used,  as  will  be  shown  in  a  later  paragraph. 


B_ 

D 

'r\. 

A 
A' 

■_ —^==* 

B 

§___ 

Wf-^r^S^^^^^ 

a'' 

^ 

A 

VJ 

U^- 

Figure  26 

Figure  27  is  a  section  of  a  terrestrial  telescope  much 
used  for  military  purposes.  Glasses  of  this  type  are 
quite  generally  known  as  ^^ spyglasses.'^ 

As  in  the  case  of  the  astronomical  telescope,  the 
first  inverted  image  ha  is  formed  at  the  focal  plane  (F), 
and  the  first  eyeglass  converges  these  pencils  to  L. 
Instead  of  placing  the  eye  at  L,  as  in  the  astronomical 


\*^rocal  planes  --*| 


Figure  27 


telescope,  the  pencils  are  allowed  to  cross  and  fall  on 
a  second  eyeglass,  by  which  the  rays  of  each  pencil  are 
converged  to  a  point  in  the  second  erect  image  a'  V , 
which  image  is  viewed  by  means  of  the  third  and  last 
eyeglass. 

Terrestrial  telescopes  have  a  comparatively  small 
field  of  view.  The  barrels  of  this  telescope  are  neces- 
sarily long  on  account  of  the  additional  lenses. 


106 


GALILEAN    FIELD    GLASSES    AND   TELESCOPES. 

Figure  28  is  a  section  of  a  Galilean  telescope  which 
differs  from  the  astronomical  telescope  in  having  a 
double  concave  instead  of  a  double  convex,  eyepiece 
or  ocular. 

In  this  telescope  the  rays  from  an  object  are  con- 
verged by  the  object  glass  (0)  and  would  normally 
focus  at  the  focal  plane  {C)  and  there  form  the  inverted 
image  ha  were  it  not  that  the  double  concave  eyeglass 
or  ocular  (Z>)  is  so  located  in  the  barrel  of  the  tele- 


Figure  23 


scope  as  to  intercept  the  pencils  before  they  are 
focused.  This  double  concave  eyeglass  diverges  these 
pencils  and  forms  a  magnified  erect  image  a'  &' 
apparently  at  E.  Due  to  the  diverging  action  of  this 
concave  eye  lens,  the  cone  of  pencils  entering  the  eye 
is  larger  than  the  pupil  of  the  eye,  and  therefore  but  a 
small  part  of  the  field  gathered  by  the  object  glass  is 
utilized  by  the  eye,  which  causes  telescopes  of  this 
type  to  have  a  comparatively  small  field  of  view. 

PORRO    PRISM  FIELD  GLASSES    AND   TELESCOPES. 

In  1850  a  French  engineeer,  Porro,  discovered  a 
combination  of  prisms  which,  when  inserted  between 
the  objective  and  the  eyepiece  of  an  astronomical  tele- 


107 

scope,  showed  the  image  erect  or  in  its  natural  posi- 
tion, while  the  same  telescope  without  the  prisms 
showed  the  image  inverted.  Practical  use  of  this  dis- 
covery was  not  made  for  many  years  after.  These 
prisms  served  a  twofold  purpose,  viz,  showing  the 
image  of  the  object  looked  at  in  its  natural  position 
instead  of  reversed,  and  second,  the  shortening  of  the 
telescope  by  twice  turning  the  ray  of  light  upon  itself. 
Each  tube  of  the  prism  field  glass  contains  two  of  these 
double-reflecting  prisms.  The  ray  of  light  passing 
through  the  object  glass  enters  the  first  prism  in  such 
a  manner  as  to  be  twice  totally  reflected,  each  time  at 
an  angle  of  90°,  thus  emerging  parallel  to  the  entering 
ray,  but  in  the  opposite  direction.  It  is  thus  caught 
by  the  second  prism  and  is  similarly  reflected  and  sent 
on  its  original  direction  without  change  except  in  one 
very  important  point,  viz,  the  image  of  the  object 
observed,  which,  )vithout  the  intervention  of  the 
prism,  would  be  upside  down,  is  now  erect,  and  will  be 
magnified  by  the  simple  astronomical  eyepiece  just  as 
the  stars  and  planets  are  magnified  in  large  telescopes. 

The  field  of  view  of  the  Porro  prism  glass  is  consid- 
erably larger  than  that  of  the  ordinary  field  glass.  It 
decreases  about  12^  per  cent  with  each  magnifying 
power,  a  number  6-power  glass  giving  a  linear  view  of 
118  feet  in  a  thousand,  while  in  a  number  10  glass  the 
field  is  but  70  linear  feet.     This  is  explained  as  follows : 

The  rays  of  light  emerging  from  the  ocular  of  the 
Galilean  telescope  are  divergent  and  cover  an  area 
much  greater  than  the  size  of  the  pupil  of  the  eye.  As 
all  rays  falling  outside  the  pupil  of  the  eye  are  lost,  but 
a  small  field  of  view  can  be  seen,  as  in  looking  through 


108 

an  ordinary  cone  from  the  larger  end.  The  prism 
glasses  are  constructed  on  the  opposite  principle. 
The  rays  of  light  gathered  by  the  objective  emerge 
from  the  eyepiece  in  a  converging  pencil  of  light  small 
enough  to  enter  the  pupil  of  the  eye,  thus  giving  a 
larger  field  of  view;  theoretically,  nine  times  the  area 
given  by  the  old-style  instrument  of  the  same  power. 
With  these  advantages,  however,  the  Porro  prism 
glass  has  not  been  found  in  all  respects  satisfactory 
for  field  service.     With  a  clear  atmosphere  and  the 

object  which  is  be- 
ing   viewed    well 
O.   ^  illuminated,    it    is 

\\^  distinctly  superior 

to    the   Galilean 
field-type  glass  in 
respect     to    light, 
^     ^    ^         .  power,  and  defini- 

Fio.  29.— Porro  prism.  ^.  ' 

tion.  The  prisms 
having  once  been  deranged,  however  slightly,  satisfac- 
tory use  of  the  glass  can  not  be  had  until  the  prisms 
have  been  readjusted,  and  until  very  recently  it  was 
impracticable  to  have  this  done  elsewhere  than  at  the 
place  of  manufacture  of  the  glass. 

FIELD    GLASSES. 

The  field  glass  or  binocular  is  a  combination  of  two 
similar  telescopes  and  possesses  mechanical  adjust- 
ments capable  of  focusing  the  two  telescopes  simul- 
taneously or  separately,  depending  upon  the  type 
considered. 

Field  glasses  are  divided  into  two  general  classes, 
viz,  the  Galilean  glasses  and  the  Porro  prism  glasses. 


109 

PROPERTIES   OF   TELESCOPES   AND   FIELD   GLASSES. 

Telescopes  and  field  glasses  have  four  properties, 
viz,  power,  light,  field,  and  definition.  These  prop- 
erties are  expressed  in  terms  of  the  corresponding 
qualities  of  the  unaided  eye. 

Eyes  are  of  very  different  capabilities.  Some 
people  have  '^ short''  sight  while  others  have  ^^far" 
sight.  There  are  normal,  excellent,  and  weak  eyes. 
In  the  following  discussion  the  capabilities  of  the 
normal  eye  are  assumed. 

For  each  individual  there  is  a  certain  distance  at 
which  objects  may  be  most  distinctly  seen.  This 
is  called  the  ^Visual  distance."  With  shortsighted 
eyes  this  distance  is  from  3  to  6  inches;  with  normal 
eyes,  from  8  to  14  inches,  and  with  farsighted  eyes, 
from  16  to  28  inches. 

The  capabilities  of  the  normal  unassisted  eye  may 
therefore  be  expressed  as  follows:  Power,  1;  light,  1; 
field,  45°;  definition,  40'' to  3'. 

Power, — At  the  ^Visual  distance,"  all  objects  seen 
by  the  unaided  normal  eye  appear  in  their  natural 
size.  At  less  than  the  ^Visual  distance"  they  appear 
indistinct,  blurred,  and  imperfectly  defined;  at  greater 
than  the  '^visual  distance"  objects  are  clear  and  well 
defined,  but  diminish  in  size,  the  more  so  as  they  are 
farther  removed. 

The  ability  of  a  lens  to  magnify  the  apparent 
diameter  of  an  object  is  termed  its  power. 

The  power  of  a  lens  is  defined  as  the  ratio  of  the 
diameter  of  the  object  as  seen  through  the  lens  to 
the  diameter  as  viewed  by  the  unaided  eye. 


110 

The  power  is  also  defined  as  the  ratio  of  the  focal 
distance  of  the  object  glass  to  that  of  the  eyepiece. 

The  power  of  a  field  glass  can  be  roughly  determined 
by  focusing  the  instrument  on  a  wall  or  a  range  rod, 
by  looking  at  the  object  through  the  instrument  with 
one  eye  and  at  the  same  object  directly  with  the 
unaided  eye.  A  comparison  of  the  diameter  of  the 
two  images  gives  the  ratio. 

The  power  of  a  telescope  or  a  field  glass  can  more 
accurately  be  measured  by  means  of  a  dynameter, 
which  is  a  microscope  which  can  be  fitted  over  the 
eyepiece  end  of  the  instrument,  and  which  magnifies 
the  image.  The  end  of  the  dynameter  next  to  the 
eyepiece  of  the  instrument  is  ruled  with  a  series  of  lines 
one-hundredth  of  an  inch  apart.  On  focusing  the 
dynameter,  the  image  of  the  emerging  pencil  appears 
as  a  sharply  defined  ring  of  light  with  the  magnified 
scale  of  the  dynameter  across  it. 

The  number  of  subdivisions  covered  by  the  diam- 
eter of  the  ring  of  light  is  noted.  The  diameter  of  the 
object  glass  is  similarly  measured  by  means  of  a  pair 
of  dividers  and  read  to  the  hundredth  part  of  an  inch. 

The  ratio  of  the  diameter  of  the  object  glass  to  that 
of  the  image  as  seen  in  the  dynameter  gives  the  power 
of  the  instrument.  This  method  is  not  applicable  in 
the  case  of  the  Galilean  telescope  or  the  field  glass 
consisting  of  two  Galilean  telescopes,  due  to  the  fact 
that  the  rays  from  the  eyepiece  of  the  Galilean  tele- 
scope are  divergent. 

Field  glasses  in  which  the  image  appears  magnified 
from  one  to  six  diameters  are  known  as  '4ow-power" 
glasses.  Field  glasses  which  produce  an  image 
magnified  over  six  diameters  are  termed  ^^ high  power.'' 


Ill 

For  the  mounted  man  a  glass  of  but  4,  or  at  most  6, 
powers,  can  be  used  with  advantage ;  on  foot,  with  free 
hand,  instruments  of  not  to  exceed  10  powers  can  be 
used.  If  more  than  10  powers  are  desired,  a  holder 
becomes  necessary,  and  if  the  holder  is  intended  to  be 
portable  a  greater  power  than  50  is  not  practicable, 
as  the  movement  of  the  air  or  the  slightest  touch  of 
the  hand  sets  up  vibrations  that  render  clear  vision 
impossible. 

Field  glasses  with  low  magnifying  power,  which  are 
usually  preferred  by  ordinary  observers,  have  their 
chief  value  in  the  comparatively  extensive  field  of  view ; 
they  should  be  used  to  observe  extensive  movements, 
where  large  tracts  of  country  must  be  taken  in  one 
field  of  view  or  in  sweeping  the  landscape  to  find  the 
tents  of  the  enemy,  their  wagons,  etc.,  or  other  objects, 
to  be  afterwards  more  closely  examined  with  the  tele- 
scope. 

They  may  be  used  on  shipboard  or  in  boats,  where 
the  rolHng  motion  interferes  with  the  use  of  the  tele- 
scope; also  on  horseback  or  in  hasty  examination 
made  on  foot  or  in  trees,  and  generally  for  all  observa- 
tions not  critical  or  those  to  be  made  under  circum- 
stances where  the  telescope  can  not  be  conveniently 
handled.  The  field  glass  ought  to  be  held  by  both 
hands  when  in  use,  and  to  steady  it  the  arms  should  be 
kept  close  to  the  body. 

For  reading  signals  at  short  ranges,  say,  up  to  5 
miles,  these  glasses  are  better  than  the  telescope. 
Flag  signals  have  frequently  been  read  with  glasses  of 
this  description  at  a  distance  of  10  miles. 

Light. — The  illumination  of  an  object  when  ob- 
served with  the  unaided  eye  is  impressed  upon  the 


112 

retina  with  a  brightness  in  strict  proportion  to  that 
of  the  object  itself.  If  an  object  be  viewed  under 
equal  illuminating  conditions  alternately  with  the 
naked  eye  and  with  a  glass,  the  brightness  ,of  the 
image  seen  with  the  naked  eye  may  be  represented 
by  1,  while  that  of  the  image  in  the  glass  will  gen- 
erally differ,  being  greater  or  less. 

The  light  of  the  telescope  or  field  glass  is  expressed 
by  the  number  which  shows  how  many  times  brighter 
the  object  appears  through  the  instrument  than  to  the 
naked  eye.  Light  is  a  function  of  the  dimensions 
of  the  object  glass  and  of  the  power  of  the  instrument, 
and  is  sometimes  determined  by  dividing  the  square 
of  the  objective  aperture  (expressed  in  milHmeters) 
by  the  square  of  the  power. 

The  light  of  a  telescope  or  field  glass  can  also  be 
determined  by  means  of  the  absorption  apparatus 
shown  in  figure  30  (a)  (&)  (c). 

This  absorption  apparatus  operates  on  the  principle 
of  viewing  an  object  through  a  perfectly  black  liquid, 
which  absorbs  all  colors  equally,  and  of  increasing  the 
thickness  of  the  liquid  layer  until  the  object  becomes 
invisible.  The  thickness  of  the  layer  of  liquid  will 
then  be  a  measure  of  the  relative  brightness  or  inten- 
sity of  the  illumination. 

The  apparatus  consists  of  two  wedge-shaped  vessels, 
made  of  brass,  with  glass  windows  in  the  sides.  One 
of  these  vessels  is  shown  in  perspective  in  figure  30a. 
The  sides  A  and  the  one  opposite  are  of  glass.  B  is 
tubulure  for  filhng  the  apparatus,  and  is  stopped  with 
a  cap.  The  operation  of  the  apparatus  is  shown  dia- 
grammatically  in  figures  30&  and  30c.     The  edges  of 


113 


the  two  wedges  which  come  together  are  divided  into 
scales  of  equal  parts  of  convenient  magnitude.     Each 


FiG.  30(a) 


FIG.    30(b) 


FIG     30(c) 


scale  begins  with  zero;  not  at  the  extreme  point  of  the 
wedge  outside,  but  at  a  point,  which,  allowing  for  the 

40422—10 8 


114 

thickness  of  the  glass  sides,  is  opposite  the  point  of  the 
wedge  of  Uquid  inside.  It  will  be  observed  in  figures 
30&  and  30c  that  the  sum  of  any  two  adjacent  num- 
bers, on  the  respective  scales,  over  the  whole  overlap- 
ping portion  of  the  wedges,  is  the  same.  Thus  in 
ifigure  306  it  is  11,  and  in  figure  30c  it  is  7.  These 
figures  measure  the  relative  thickness  of  the  liquid 
layers  in  the  two  respective  settings  of  the  apparatus. 
Suppose  the  image  is  just  obliterated,  when  looking 
with  the  unaided  eye,  at  the  setting  shown  in  figure 
306,  and  when  using  the  glass  at  the  setting  shown 
in  figure  30c.  This  would  mean  that  the  illuminating 
power  of  the  glass  is  seven-elevenths.  In  using  the 
apparatus,  a  focusing  cloth,  used  by  all  photographers, 
is  useful  in  excluding  stray  Hght. 

Field. — Maintaining  the  head  and  eyes  as  motion- 
less as  possible,  the  field  of  vision  of  the  unaided  eye 
or  the  range  within  which  objects  can  be  perceived 
by  the  unaided  eye  varies  according  to  direction. 

De  Schweinitz  gives  the  following  limits :  Outward, 
90°;  outward  and  upward,  70°;  upward,  50°;  upward 
and  inward,  55°;  inward,  60°;  inward  and  downward, 
55°;  downward,  72°;  downward  and  outward,  85°. 

It  may  be  safely  said  that  the  field  or  ^'visual  angle'' 
of  the  unaided  eye  for  distinct  vision  is  at  least  45°  in 
aU  directions. 

The  'Wisual  angle''  or  ^ Afield"  of  a  field  glass  is 
always  smaller,  no  field  glass  having  yet  been  designed 
which  could  equal  the  field  of  the  unaided  eye. 

The  field  of  a  telescope  or  field  glass  can  best  be 
determined  by  the  use  of  a  transit  or  other  instru- 
ment used  in  measuring  horizontal  angles.  The  glass 
is  placed  upon  the  telescope  of  the  transit  in  such  a 


115 

way  that  the  axes  of  coUimation  of  the  transit  and 
the  telescope  or  field  glass  are  parallel.  The  extreme 
limits  of  the  field  of  view  are  marked  and  the  horizon- 
tal angle  between  the  markers  noted  on  the  limb  of 
the  transit. 

Definition. — One  of  the  chief  qualities  of  the  eye  is 
its  power  of  defining  outlines  and  details  distinctly. 
Relative  characteristics  in  this  respect  may  be  deter- 
mined in  various  ways.  Thus  the  distance  at  which 
printed  matter  can  be  read,  or  the  details  of  a  distant 
object  distinguished,  will  give  a  fair  measure  of  the 
defining  power  of  the  eye;  but  a  better  method  is  to 
express  the  definition  of  sight  by  angular  measure- 
ment— that  is,  by  the  determination  of  the  smallest 
visual  angle  giving  clear  results.  Experience  teaches 
that  this  angle  of  the  normal  eye  (with  good  light  and 
favorable  color  conditions)  is  about  40'',  and  it  is 
therefore  possible  to  determine  the  smallest  object 
which  can  just  be  seen,  well  defined,  at  an  arbitrary  dis- 
tance. For  instance,  at  a  distance  of  15  feet  an  object 
can  be  seen  which  is  one-twentieth  of  an  inch  high  or 
broad;  at  30  feet  distance,  consequently,  the  object 
must  be  twice  the  size  (one-tenth  of  an  inch)  to  be 
seen,  and  so  on  relatively,  within  limits,  as  distance  in- 
creases. But  as  the  distance  becomes  greater  sharp- 
ness of  vision  is  impaired  materially  by  the  interposing 
atmosphere,  while  it  is  also  affected  by  color  contrasts 
and  conditions  of  illumination.  It  therefore  follows 
that  at  considerable  distances  objects  which  subtend  a 
visual  angle  of  40'' are  no  longer  clearly  defined  but 
become  so  only  as  the  angle  approaches  60",  120", 
180",  or  more. 


116 

The  most  important  and  essential  quality  of  a  tele- 
scope or  field  glass  is  definition,  i.  e.,  the  sharpness, 
clearness,  and  the  purity  of  the  images  seen  through 
it.  To  obtain  good  definition  it  is  necessary  that 
spherical  and  chromatic  aberration  be  overcome,  that 
the  polish  of  the  lenses  be  as  perfect  as  possible,  that 
the  cement  possess  no  inequalities,  and  that  the  lenses 
be  well  focused,  that  there  be  no  dampness  in  the  inte- 
rior of  the  tubes,  and,  generally,  that  the  instrument 
be  without  optical  defect. 

Faults  in  this  direction  are  discovered  at  once  by 
examination  of  definition,  whereas  in  determining  the 
other  constants  they  are  less  noticeable.  In  comparing 
the  definition  of  any  two  instruments  it  is  ordinarily 
necessary  only  to  scan  distant  objects  and  observe  to 
what  extent  details  may  be  distinguished. 

The  following  test  may  also  be  used:  Focus  on 
printed  matter  at  a  distance  just  beyond  that  at  which 
perfect  clearness  is  given  and  gradually  approach  until 
the  letters  are  distinctly  defined.  The  instrument 
with  which  the  print  can  be  read  at  the  greatest  dis- 
tance has  the  best  definition. 

To  express  definition  as  an  absolute  measure,  use 
instead  of  printed  matter,  a  white  sheet  of  paper  upon 
which  a  series  of  heavy  lines  are  drawn  at  intervals 
equivalent  to  their  thickness.  Focus  upon  this  and 
gradually  approach  from  a  point  where  the  impression 
of  a  uniform  gray  field  ceases  and  the  black  lines  and 
white  intervals  begin  to  appear  distinct  and  defined. 

Let  the  distance  thus  found  be  20  yards  and  the 
thickness  of  the  lines  and  intervals  between  them  one- 
tenth  inch.     The  circumference  of  a  circle  with  a  ra- 


117 

dius  of  20  yards  or  7,200  tenths  inches  is  14,400  by 
3.1416  or  45,240  tenth  inches;  but  a  circumference 
equals  360°  or  (360  by  60  by  60)  1,296,000''. 

If,  therefore,  45,240  tenths  inches  correspond  to 
1,296,000'',  then  1  tenth  inch  equals  1,296,000  divided 
by  45,240,  or  28.6".  The  definition  is  therefore  28.6", 
or  practically  half  a  minute. 

The  capabilities  of  glasses,  including  telescopes,  in 
a  general  way,  lie  between  the  following  limits : 

(1)  Power  between  2  and  1,000. 

(2)  Light  may  be  0.01  to  200  times  that  of  the 
unaided  eye. 

(3)  Field  measures  in  most  favorable  case,  10°;  in 
the  most  unfavorable,  .01°. 

(4)  Definition  varies  between  40"  and  0.1". 
Thus,  as  a  maximum,  an  object  may  be  seen  by 

means  of  a  telescope,  magnified  1,000  times,  200 
times  brighter  and  400  times  sharper  than  with  the 
naked  eye. 

If  these  advantages  could  be  fully  utilized  for  mili- 
tary purposes  the  use  of  glasses  would  be  extraordi- 
nary, a  power  of  1,000  practically  effecting  the  same 
purpose  as  the  approach  of  the  observed  object  to 
one-thousandth  of  the  distance.  A  hostile  command 
10  miles  distant  could  be  seen  theoretically  as  well  as 
if  they  were  only  53  feet  away,  and  the  slightest 
movement  of  each  single  man  would  become  visible. 
Of  course  no  such  wonderful  effect  is  physically  prac- 
ticable, and  the  limiting  conditions  increase  greatly  in 
proportion  as  either  one  or  the  other  of  the  qualities, 
power,  field,  etc.,  is  especially  sought. 


118 

While  astronomers  require  only  that  the  telescope 
be  made  as  capable  and  perfect  as  possible  in  an  optical 
point  of  view,  making  all  other  conditions  subordinate 
to  this  one,  the  military,  to  whom  the  glass  is  simply 
an  accessory,  make  other  conditions  of  the  first  impor- 
tance. The  glass  must  have  suitable  form,  small  vol- 
ume, little  weight,  and  that  it  may  be  used  without 
support,  mounted  or  dismounted,  and  the  image  must 
appear  as  looked  at  by  the  naked  eye — that  is,  not 
inverted. 

The  capability  of  the  instrument,  however,  is  there- 
by much  limited;  great  powers  give  plain  images  only 
with  relatively  long  tubes;  glasses  must  be  held  the 
steadier  the  more  they  magnify;  and  with  increasing 
power  all  vibrations  become  more  troublesome  and 
render  minute  observations  very  difficult  or  impossible. 
The  additional  lenses  in  terrestrial  telescopes  some- 
what decrease  power  and  affect  also  light  and  defini- 
tion. It  is  clear  therefore  that  expectations  of  achiev- 
ing great  power  should  not  be  entertained,  the  function 
of  field  glasses  being  to  bring  out  and  define  objects 
which  to  the  naked  eye  appear  indistinct  and  doubtful. 

The  distinctness  with  which  anything  can  be  seen 
through  the  telescope  depends,  primarily,  upon  the 
number  of  straight  lines  of  light  which  are  collected 
by  it  from  every  point  of  the  object. 

Telescopes,  the  object  glasses  being  equal  in  size, 
diminish  light  as  a  general  rule  in  proportion  as  their 
magnifying  power  is  great.  The  most  powerful 
glasses  are  therefore  to  be  used  for  minute  observa- 
tions on  the  clearest  days  or  when  there  is  a  strong 
fight  upon  the  observed  object.     When  the  light  is 


119 

fading  or  there  is  a  little  light  upon  the  observed 
object  the  clearer  view  will  be  had  with  glasses  of 
large  field  and  low  magnifying  power. 

FIELD  GLASSES  AND  TELESCOPES  ISSUED  BY  THE  SIGNAL 
CORPS. 

The  Signal  Corps  issues  four  standard  field  glasses, 
viz,  Type  A,  Type  B,  Type  C,  Type  D. 

Field  glasses  issued  by  the  Signal  Corps  are  not  sup- 
phed  for  the  personal  use  of  an  officer  and  will  not  be 
used  in  lieu  of  the  officer^  s  personal  field  glass  pre- 
scribed by  paragraph  97,  General  Orders,  169,  War 
Department,  1907  (Par.  1,  G.  O.  16,  War  Dept.,  1910). 

Under  paragraph  1582,  Army  Regulations,  as 
amended  by  paragraph  I,  General  Orders,  No.  207, 
War  Department,  October  16,  1909,  the  Signal  Corps 
will  sell  field  glasses  to  officers  of  the  army  for  their 
personal  use. 

Application  for  the  purchase  of  field  glasses  should 
be  addressed  to  the  Chief  Signal  Officer  of  the  Army, 
Washington,  D.  C,  inclosing  post-office  money  order 
or  check  on  the  Treasurer  or  Assistant  Treasurer  of 
the  United  States  for  the  amount,  payable  to  the  Dis- 
bursing Officer,  Signal  Corps,  and  Signal  Corps  Form 
No.  240  accomplished  in  duplicate. 

The  Government  does  not  pay  transportation 
charges  for  the  shipment  of  articles  sold  to  officers. 
Field  glasses  are  sent  from  the  Signal  Corps  General 
Supply  Depot,  Fort  Wood,  New  York  Harbor,  by 
express,  charges  collect,  unless  purchase  request  is 
accompanied  by  funds  so  that  field  glasses  may  be 
sent  by  registered  mail.     Forwarding  by  registered 


120 

mail  is  somewhat  cheaper  than  by  express,  and  the 
amount  of  postage  required  is  40  cents  for  Type  D 
glass,  46  cents  for  Types  A  and  B,  and  74  cents  for 
Type  C.  Express  charges  depend  upon  the  distance 
from  New  York. 

The  Signal  Corps  has  purchased  many  samples  of 
field  glasses  from  various  manufacturers  with  a  view 
of  testing  their  suitability  for  the  military  service. 
These  samples  may  be  examined  by  officers  of  the 
army  at  the  signal  office  in  Washington.  Among 
these  samples  there  are  many  excellent  glasses  espe- 
cially suitable  for  the  military  service,  but  the  higher 
grades  are  too  expensive  for  general  issue  to  line 
organizations  in  large  quantities.  Officers  desiring 
an  especially  fine  field  glass  should  inspect  the  sam- 
ples referred  to;  these,  however,  are  not  for  sale  by 
the  Government,  but  information  will  be  supplied 
concerning  dealers  and  cost. 

No  advice  or  fixed  rule  can  be  stated  as  to  what  con- 
stitutes the  most  suitable  characteristics  of  a  field 
glass.  No  single  field  glass  can  furnish  maximum 
results  under  all  conditions  on  account  of  varying 
conditions  of  the  atmosphere. 

A  high-power  glass  is  unsuitable  for  use  at  night, 
hazy  atmosphere,  or  for  use  of  a  mounted  man  where 
the  glass  can  not  be  rested  against  a  firm  support.  A 
low-power  glass  with  large  object  lens  to  permit  as 
much  light  as  possible  is  a  necessary  condition  for  use 
at  night.  The  double  power  glass  which  is  issued  as  a 
part  of  the  visual  signaling  outfits  was  designed  for 
the  military  service  as  a  compromise  for  conflicting 
conditions. 


121 

A  brief  description  of  the  field  glasses  issued  by  the 
Signal  Corps,  together  with  the  cost  of  the  same,  is 
given  below. 

Type  A: 

This  glass  is  the  current  result  of  the  efforts  of  the 
Signal  Corps  to  provide  a  field  glass  that  will  meet 
the  greatest  variety  of  conditions,  and  insure  efficient 


Fig.  31.— Type  A.    Showing  the  field  glass  and  case  with  sling  cord,  shoulder 
straps,  belt  loops,  and  compass. 

service  to  the  greatest  number  of  military  observers. 
It  is  really  two  glasses  in  one — a  day  glass  of  medium 
power,  and  a  nigJit  glass  of  low  power. 

It  is  to  be  clearly  understood  that  while  this  glass 
is  considered  superior  for  moderate  ranges,  it  does  not 
replace,  under  special  conditions,  for  long  ranges, 
either  the  porro  prism  glass  or  the  telescope. 


122 

When  held  as  shown  in  figure  32  with  the  tubes 
drawn  out  about  1  inch  to  secure  proper  focus,  the 
glass  has  a  power  of  about  5.6  diameters,  and  a  field 
of  about  5.4  degrees. 

If  the  glass  is  turned  into  the  position  shown  in  fig- 
^ure  33,  the  small  plus  lenses,  just  in  front  of  the  eye 
pieces,  drop  automatically  into  position  and  reduce 
the  power  to  3.8  diameters,  and  increase  the  field  to 


Fig.  32.— Signal  Corps  field  glass,  Type  A. 

8.3  degrees.  This  position  requires  a  different  adjust- 
ment, the  tubes  being  drawn  out  about  one- third  of 
an  inch  to  get  the  proper  focus.  It  will  be  observed 
in  the  illustrations  that  the  rear  bar  of  the  frame  is 
not  only  lettered  to  indicate  wliich  power  is  being 
used,  but  the  bar  itself  is  shaped  with  a  hump  on  one 
side,  and  hollowed  on  the  other.     When  the  hump  is 


123 

up,  the  low  power  is  in  use.  This  is  to  faciUtate 
adjustment  in  the  dark. 

The  action  of  the  small  automatic  lenses  is  free  and 
positive.  Neither  the  eyepieces  nor  the  sections  contain- 
ing the  small  lenses  should  he  unscrewed^  except  in  case 
of  necessity,  and  then  not  hy  unsTcilled  hands. 

The  frame ^  of  aluminum  and  brass,  is  composite, 
to  give  lightness  and  strength;  and  while  it  is  con- 


FiG.  33.— Signal  Corps  field  glass,  Type  A. 

structed  to  withstand  the  rough  handling  of  field  serv- 
ice, no  field  glass  is  proof  against  careless  or  wanton 
treatment.  The  tubes  are  covered  with  tan  leather, 
and  a  round  sling  cord,  braided  from  four  strands  of 
pliable  tan  leather,  is  fastened  by  snaps  to  eyes  in  the 
frame. 


124 

The  case  is  of  tan  calfskin,  provided  with  shoulder 
strap,  and  has  an  efficient  small  compass  set  into  the 
cover.  Two  loops  are  sewed  to  the  back  of  the  case 
so  that  it  may  be  worn  on  a  belt. 

The  glass,  complete  with  case,  cord,  and  straps, 
weighs  21.5  ounces. 

Two  of  these  glasses  are  issued  to  each  company  of 
infantry  and  coast  artillery,  Philippine  Scouts,  and 
Signal  Corps,  and  to  each  troop  of  cavalry  for  use  in 
instruction  in  visual  signaling.  Below  is  a  brief 
description  of  the  type  A  glass. 

Magnification,  3^  and  5^  diameters;  Galilean  type; 
object  lens,  1^  inches;  tan  leather  finish;  tan  leather 
carr3ring  case  with  compass;  weight  of  glass,  complete, 
with  case,  cord,  and  strap,  25  ounces.  At  a  distance 
of  1,000  yards  the  field  of  view  includes  a  diameter  of 
123  yards  for  the  3J  power,  and  73  yards  for  the  5^ 
power.  Length  of  glass  closed,  4  inches.  This  glass 
is  issued  as  a  part  of  the  visual  signaling  kit  to  each 
company  of  infantry,  coast  artillery,  and  Philippine 
Scouts,  troop  of  cavalry,  machine-gun  platoon,  and 
Signal  Corps  field  company.     Price,  $12.15. 

The  latest  issue  of  this  glass  known  as  the  Type 
A,  model  1910,  includes  provision  for  interpupillary 
adjustment,  the  two  barrels  being  hinged  to  accom- 
modate the  glass  to  the  distance  between  the  pupils  of 
the  eye.     The  price  of  the  model  1910  glass  is  $14.75. 

Type  B: 

This  field  glass  is  similar  in  appearance  and  con- 
struction to  the  Type  A  glass,  and  is  issued  to  the 
field  artillery  organizations  upon  requisition.  The 
following  is  a  brief  description : 


125 

Magnification,  4^  and  6^  diameters;  Galilean  type; 
object  lens,  If  inches;  interpupillary  adjustment;  tan 
leather  finish;  tan  leather  carrying  case  with  compass; 
weight  of  glass,  complete,  with  case,  cord,  and  straps, 
26  ounces;  length  of  glass  closed,  4^  inches.  At  a 
distance  of  1,000  yards  the  field  of  view  includes  a 
diameter  of  90  yards  for  the  4J  power,  and  60  yards 
for  the  6i  power.  This  glass  is  issued  as  a  part  of  the 
fire-control  equipment  to  field  artillery.     Price,  $17.50. 

TypeC: 

The  type  C  is  a  high  power  glass  of  the  porro  prism 
type  and  is  issued  only  to  certain  organizations  of  the 
field  artillery,  Signal  Corps,  and  to  all  machine-gun 
platoons. 

Description. — Magnification,  10  diameters;  pris- 
matic type;  object  lens,  If  inches;  interpupillary 
adjustment;  tan  leather  finish ;  sunshade;  tan  leather 
carrying  case;  weight  of  glass,  complete,  with  case, 
cord,  and  straps,  46  ounces;  length  of  glass  closed, 
7f  inches.  At  a  distance  of  1,000  yards  the  field  of 
view  includes  a  diameter  of  80  yards.  This  glass  is 
issued  to  reconnaissance  officers  of  field  artillery. 
Price,  $39.90. 

Type  D:  Purchase  has  been  made  for  delivery  in 
the  near  future  of  a  supply  of  a  new  type  of  high 
power  prismatic  field  glass  for  sale  and  issue.  This 
new  type  of  glass,  to  be  known  as  type  D,  is  consider- 
ably smaller  than  the  type  C  glass,  as  is  shown  by 
figure  34.  The  glass  in  a  tan-colored  carrying  case 
weighs  15  ounces,  the  field  glass  without  the  case 
weighing  but  9  ounces.  The  magnification  is  8 
powers  and  the  field  of  view  (with  both  eyes)  5°  40'. 
The  estimated  cost  will  be  $27. 


126 

TELESCOPES    ISSUED    BY   THE    SIGNAL   CORPS. 

Type  A:  This  glass  complete  consists  of  a  2-iiich 
prism  terrestrial  telescope,  powers  18  and  24,  with 
alt-azimuth,  folding  tripod,  and  carrying  case. 

TypeB:  This  telescope  is  a  19-27  power,  2-draw 
terrestrial  telescope,  in  leather  carrying  case  with 
sling.  The  leather  carrying  case  also  includes  a  holder 
which  can  be  screwed  into  a  tree,  post,  or  other  sta- 
tionary wooden  object. 

GENERAL  SPECIFICATION  NO.  263. 

llevised  February  10, 1910.] 
SERVICE   FIELD   GLASSES. 

1.  Preliminary. — This  specification  covers  the  de- 
sign and  construction  of  field  glasses,  types  A  and  B, 
each  having  two  powers  as  hereinafter  specified. 

2.  Sample. — The  bidder  shall  furnish  with  his  pro- 
posal a  sample  of  the  glass  which  he  will  supply,  and 
award  will  be  made  after  comparison  of  the  samples 
with  models  on  file  in  the  office  of  the  Chief  Signal 
Officer.  The  maker  will  be  allowed  to  examine  the 
model  glasses  in  detail  in  the  office  of  the  Chief  Signal 
Officer  of  the  Army,  Washington,  D.  C. 

3.  Inspection  and  test. — When  the  order  under  this 
specification  is  complete,  the  contractor  will  notify 
the  Chief  Signal  Officer  of  the  Army,  who  will  cause 
an  inspection  to  be  made.  It  shall  be  the  duty  of  the 
contractor  to  remedy  any  defects  pointed  out  by  the 
inspector,  and  the  contractor  will  be  held  accountable 
for  any  imperfections  which  the  inspector  may  have 
overlooked. 


127 


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128 

The  Chief  Signal  Officer  of  the  Army  reserves  the 
right  to  inspect  any  or  all  processes  of  manufacture, 
and  unsatisfactory  material  will  be  marked  for  rejec- 
tion by  the  inspector  before,  during,  or  after  assembly, 
as  occasion  may  arise. 

Each  glass  will  be  tested  for  power,  field,  definition, 
and  light.  Any  glass  which  is  not  the  equal  of  the 
sample  and  model  in  all  respects  will  be  rejected. 
The  properties  above  enumerated  will  be  tested  as 
follows : 

(a)  Power:  In  testing  for  power  the  glass  will  be 
placed  upon  a  firm  support  about  the  height  of  the 
eye  and  directed  upon  a  range  rod,  accurately  divided 
into  divisions  of  1  foot,  with  alternate  divisions 
colored  red  and  white,  respectively.  The  rod  should 
be  placed  approximately  100  feet  from  the  glass  in  a 
good  light  and  with  strongly  contrasted  background. 

The  rod  is  observed  through  the  glass  with  one  eye 
and  at  the  same  time  with  the  other  eye  unaided.  An 
accurate  comparison  of  the  two  images  by  means  of 
the  rod  scale  determines  the  magnifying  power  of  the 
glass. 

(6)  Field:  The  field  will  be  determined  by  the  use 
of  a  transit  or  any  other  instrument  adapted  to  the 
measurement  of  horizontal  angles.  The  glass  will  be 
placed  upon  the  telescope  of  the  transit  in  such  a  way 
that  the  axes  of  colhmation  of  the  telescope  and 
field  glass  barrels  are  parallel.  The  extreme  limits  of 
the  field  of  view  of  the  glass  are  marked  in  a  conven- 
ient way  and  the  horizontal  angle  of  view  accurately 
measured  with  the  transit. 


129 

(c)  Definition:  In  determining  the  definition  of  the 
glass  expressed  in  units  (seconds)  a  target  will  be  pro- 
vided with  a  number  of  lines  one-tenth  inch  thick 
with  one-tenth  inch  spaces  between  them  drawn  on 
a  piece  of  heavy  white  paper. 

At  a  certain  distance  this  target  will  appear  uni- 
formly gray  when  viewed  through  the  glass. 

The  inspector  will  gradually  approach  the  target, 
focusing  the  glass  until  he  reaches  the  most  distant 
point  from  the  target  where  the  uniform  field  ceases 
and  the  black  and  white  intervals  appear  distinct  and 
defined. 

Assume  the  distance  thus  found  to  be  20  yards  and 
the  thickness  of  the  lines  and  intervals  between  them 
one-tenth  inch.  The  circumference  of  a  circle  with  a 
radius  of  20  yards  or  7,200  tenths  inches  is  14,400  by 
3.1416,  or  45,240  tenths  inches;  but  a  circumference 
equals  360°,  or  (360  by  60  by  60)  1,296,000  seconds. 

If,  therefore,  45,240  tenths  inches  correspond  to 
1,296,000  seconds,  then  one-tenth  inch  equals  1,296,000 
divided  by  45,240,  or  28.6  seconds.  The  definition  is 
therefore  28.6  seconds,  or  practically  half  a  minute. 

The  definition  should  be  as  follows: 

For  6.5  power  glass 30  seconds. 

For  5.5  power  glass 35  seconds. 

For  4.5  power  glass 40  seconds. 

For  3.5  power  glass 55  seconds. 

(d)  Light:  The  light  of  a  field  glass  is  expressed  by 
a  number  which  is  the  ratio  of  the  amount  of  light 
which  reaches  the  eye  through  the  glass  to  the  amount 
which  enters  the  eye  unaided.  This  comparison  will 
be  reached  by  means  of  the  absorption  apparatus 

40422—10 9 


130 

furnished  by  the  Signal  Corps.  This  apparatus  con- 
sists of  two  wedge-shaped  vessels  made  of  brass  with 
glass  windows  in  the  sides,  and  are  filled  with  a  per- 
fectly black  liquid.  The  sky  line  is  first  viewed 
through  the  apparatus  with  the  naked  eye  and  the 
instrument  adjusted  to  limit  of  visibility.  The  read- 
ing of  the  scale  is  then  noted.  The  sky  line  is  again 
observed,  using  the  glass,  but  in  other  respects  as 
before,  and  a  second  scale  reading  obtained.  The  ratio 
of  these  readings  measure  the  illuminating  power  of  the 
glass  whirh  must  conform  to  the  standard  sample. 

4.  Service  field  glass^  type  A. — (a)  This  glass  shall 
conform  in  general  to  the  model,  now  on  file  in  the 
office  of  the  Chief  Signal  Officer  at  Washington.  The 
arrangement  for  changing  automatically  from  the  low 
power  to  the  high  power,  and  vice  versa,  by  the  inter- 
position of  the  plus  lens  at  the  proper  distance  in  front 
of  the  eyepiece,  must  be  strictly  adhered  to. 

(&)  The  low  power  shall  be  approximately  3^  diame- 
ters and  the  high  power  shall  be  approximately  5^ 
diameters.  The  figure  of  merit  given  by  multiplying 
the  numbers  of  diameters  power  by  the  number  of 
degrees  of  field  will  be  considered  in  the  examination 
of  samples,  along  with  the  other  properties  of  light, 
sharpness  of  definition,  and  general  excellence. 

(c)  The  tubes  J  frame  j  and  metal  fittings  shall  be  of 
aluminum  or  an  aluminum  alloy,  with  the  exception 
that  such  metal  parts  as  in  the  opinion  of  the  maker 
require  greater  strength  may  be  made  of  brass. 

Tubes  shall  be  held  firmly  in  the  frame,  single  draw, 
the  draw  action  to  be  through  a  bearing  surface  of  at 
least  five-eighths  of  an  inch  of  best  black  felt,  per- 
fectly fitted  so  as  to  preserve  perfect  alignment. 


131 

The  exterior  metal  parts,  except  where  leather 
covered,  must  be  given  the  best  and  most  durable, 
lusterless  black  finish.  The  tubes  and  shades  will  be 
neatly  covered  with  best  quality  tanned  calfskin,  the 
leather  to  be  sewed  on,  and  the  seams  to  lie  flat  next 
to  the  focusing  standard. 

The  interior  of  all  parts  to  be  painted  a  perfectly 
dead  black. 

The  sunshades,  when  drawn  out,  shall  project  at 
least  five-eighths  of  an  inch  and  not  over  1  inch 
beyond  the  edge  of  the  cell. 

The  focusing  screw  and  standard  should  follow 
closely  that  of  the  sample,  except  that  the  milled 
focusing  disk  should  have  a  face  as  nearly  one-half 
inch  wide  as  possible  and  the  milling  should  be 
sharper. 

In  addition  to  the  diaphragm  upon  which  the  auto- 
matic lens  is  mounted,  there  shall  be  two  diaphragms 
in  each  tube,  so  situated  and  so  proportioned  as  to  cut 
off  all  stray  light  and  all  internal  reflections. 

The  crossbar  supporting  the  draw  tubes  should  be 
shaped  and  engraved  exactly  as  found  in  the  model. 

{d)  The  lenses  must  be  entirely  free  from  mechan- 
ical defects,  such  as  specks,  air  bubbles,  etc. ;  must  be 
free  from  interior  strain,  and  must  be  ground  from  the 
best  obtainable  glass  for  the  purpose,  selected  for 
general  transparency,  as  colorless  as  possible,  per- 
fectly ground  and  polished,  and  accurately  centered. 

The.  object  lenses  shall  be  composite,  achromatic, 
and  well  corrected  for  spherical  aberration,  with  a 
clear  aperture  of  at  least  1^  inches,  and  not  exceeding 
1|  inches.  Bidders  will  state  the  number  and  shape 
of  the  pieces  used  to  make  up  this  lens. 


132 

The  compound  lenses  may  be  either  cemented 
together  with  Canada  balsam,  or  left  uncemented,  as 
the  maker  may  deem  best  for  durability  and  optical 
performance,  but  if  left  uncemented  the  components 
shall  have  a  permanent  mark  to  indicate  their  proper 
positions  in  the  cell. 

The  eyepieces  shall  consist  of  a  single  double  con- 
cave lens  having  a  clear  aperture  of  not  less  than  three- 
eighths  of  an  inch  and  not  more  than  one-half  of  an 
inch. 

(e)  The  sling  cord  attached  to  eyes  in  the  frame  by 
means  of  brass  snaps  with  black  burned  finish  shall 
be  round  and  braided  from  four  strands  of  pliable  tan 
leather,  and  shall  have  a  diameter  of  at  least  one- 
eighth  of  an  inch  and  not  over  one-sixth  of  an  inch. 

(/)  The  case  and  strap  must  be  exactly  like  sample, 
and  of  No.  1  stock.  Care  must  be  taken  to  put  in  only 
compasses  that  are  in  perfect  condition.  The  strap 
buckle  must  be  of  brass.  The  glass,  when  closed, 
must  not  exceed  4  inches  in  length,  and  the  glass,  case, 
cord,  and  strap,  complete,  must  not  exceed  25  ounces 
in  weight. 

(g)  The  frame  shall  be  constructed  with  jointed 
bars  for  interpupillary  adjustment. 

5.  Service  field  glass,  type  B. — {a)  The  requirements 
of  part  4,  service  field  glass,  type  A,  of  this  specifica- 
tion, shall  be  followed  in  the  design  and  construction 
of  the  type  B  glass  in  so  far  as  applicable. 

(6)  Power:  The  lower  power  shall  be  approxi- 
mately 4^  and  the  high  6^  diameters. 

(c)  Object  lenses:  These  shall  have  a  clear  aperture 
of  at  least  If  inches  diameter. 


133 

{d)  Case :  Case  and  carrying  strap  shall  be  furnished 
as  required  in  part  4  of  this  specification. 

{e)  This  glass  shall  be  constructed  with  jointed  bars 
for  interpupillary  adjustment. 

(/)  The  sunshade,  when  drawn  out,  shall  project 
not  less  than  three-eighths  of  an  inch  and  not  more 
than  1  inch  beyond  the  edge  of  the  cell. 

6.  Marking. — Glasses  furnished  under  this  specifi- 
cation shall  be  marked  on  one  barrel  with  the  words 
^^  Signal  Corps,  U.  S.  Army/'  and  on  the  other  barrel 

'^Serial  No. ^     Serial  numbers  will  be  furnished 

with  the  order.  If  not  furnished  the  contractor  at 
the  time  the  order  is  placed,  the  Disbursing  Ofl&cer 
of  the  Signal  Corps  should  be  called  upon  for  same, 
and  the  numbers  and  other  marking  placed  on  the 
glasses  prior  to  the  delivery  of  the  order. 

James  Allen, 
Brigadier-Generalj 
Chief  Signal  Officer  of  the  Army. 

vSiGNAL  Office, 

Electric  and  Telegraph  Division. 

O 


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